SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
Dec. 31, 2025
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements reflect the operations of Alnylam and our wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated. Certain prior period amounts in the consolidated financial statements have been reclassified to conform to the current period presentation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. In our consolidated financial statements, we use estimates and assumptions related to our inventory valuation and related reserves, clinical accruals, liabilities related to the sale of future royalties and development funding, income taxes, deferred tax asset valuation allowances, revenue recognition, research and development expenses, and stock-based compensation expense. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable. Actual results could differ from those estimates. Changes in estimates are reflected in reported results in the period in which they become known.
Liquidity	Liquidity Based on our current operating plan, we believe that our cash, cash equivalents, marketable securities, as well as the revenue we expect to generate from product sales and under our existing collaborations, including royalties on sales of Leqvio and Qfitlia, and available borrowing capacity under the revolving credit agreement, or the Revolving Credit Agreement, as of December 31, 2025, will be sufficient to satisfy our near-term capital and operating needs for at least the next 12 months from the filing date of this Annual Report on Form 10-K.
Concentrations of Credit Risk and Significant Customers	Concentrations of Credit Risk and Significant Customers Financial instruments that potentially expose us to concentrations of credit risk primarily consist of cash, cash equivalents and marketable securities. As of December 31, 2025 and 2024, substantially all of our cash, cash equivalents and marketable securities were invested in money market funds, certificates of deposit, commercial paper, corporate notes, U.S. government-sponsored enterprise securities and U.S. treasury securities through highly rated financial institutions. Corporate notes may also include foreign bonds denominated in U.S. dollars. Investments are restricted, in accordance with our investment policy, to a concentration limit per issuer.
Fair Value Measurements	Fair Value Measurements The fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair values determined by Level 2 inputs utilize data points that are observable, such as quoted prices (adjusted), interest rates and yield curves. Fair values determined by Level 3 inputs utilize unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. The fair value hierarchy level is determined by the lowest level of significant input.
Investments in Marketable Securities and Cash Equivalents	Investments in Marketable Securities and Cash Equivalents We invest our excess cash balances in marketable debt securities and classify our investments as either trading, held-to-maturity or available-for-sale based on facts and circumstances present at the time we purchased the securities. As of December 31, 2025 and 2024, we classified all of our investments in debt securities as available-for-sale and as current assets on the consolidated balance sheets as they represent the investment of funds available for current operations. We report available-for-sale debt securities at fair value at each balance sheet date, for which fair value measurement data is obtained from independent pricing services, and include any unrealized holding gains and losses (the adjustment to fair value) in accumulated other comprehensive loss. Realized gains and losses are determined using the specific identification method and are included in other income (expense), net. If any adjustment to fair value reflects a decline in the value of the marketable debt securities, we consider all available evidence to evaluate if an impairment loss exists, and if so, mark the investment to market through a charge to our consolidated statements of operations and comprehensive income (loss). We did not record any impairment charges related to our marketable debt securities during the years ended December 31, 2025, 2024 or 2023. Our marketable debt securities are classified as cash equivalents if the original maturity, from the date of purchase, is 90 days or less, and as marketable debt securities if the original maturity, from the date of purchase, is in excess of 90 days. Our cash equivalents are generally composed of commercial paper, corporate notes, U.S. government-sponsored enterprise securities, U.S. treasury securities, money market funds and certificates of deposit. We measure marketable equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of an investee), which have readily available prices, at fair value with changes in fair value recognized in other income (expense), net on our consolidated statements of operations and comprehensive income (loss).
Accounts Receivable, Net	Accounts Receivable, Net We record accounts receivable net of prompt payment discounts and chargebacks based on contractual terms. As of December 31, 2025 and 2024, based on our estimation of expected write-offs, we determined an allowance for doubtful accounts was not material. We have standard payment terms that generally require payment within approximately 30 to 90 days. Accounts receivable, net on our consolidated balance sheets also includes collaboration receivables and royalty receivables.
Inventory	Inventory Inventory is measured at the lower of cost or estimated net realizable value and classified based on the anticipation of when it will be consumed either within our normal operating cycle (short-term) or beyond (long-term). We use a standard cost basis, which approximates cost determined on a first-in, first-out basis. Inventory costs include all raw materials, direct conversion costs and overhead. Raw and intermediate materials that may be used for either research and development or commercial purposes are classified as inventory until the material is consumed or otherwise allocated for research and development. If the material is used for research and development, it is recorded as research and development expenses once that determination is made. We capitalize inventory costs that are expected to be sold commercially once we determine it is probable that the inventory costs will be recovered through commercial sale based on the review of several factors, including (i) the likelihood that all required regulatory approvals will be received, considering any special filing status, (ii) the expected timing of validation (if not yet completed) of manufacturing processes in the associated facility, (iii) the expected expiration of the inventory, (iv) logistical or commercial constraints that may impede the timely distribution and sale of the product, including transport requirements and reimbursement status, (v) current market factors, including competitive landscape and pricing, (vi) threatened or anticipated litigation challenges, (vii) history of approvals of similar products or formulations, and (viii) U.S. Food and Drug Administration, or FDA, (or other appropriate regulatory agencies) correspondence regarding the safety and efficacy of the product. Prior to the capitalization of inventory costs, we record such costs as research and development expenses on our consolidated statements of operations and comprehensive income (loss). We reduce our inventory to net realizable value for potentially excess, dated or obsolete inventory based on our quarterly assessment of the recoverability of our capitalized inventory. We periodically review inventory levels to identify what may expire prior to expected sale or has a cost basis in excess of its estimated realizable value and write down such inventories as appropriate.
Property, Plant and Equipment, Net	Property, Plant and Equipment, Net Property, plant and equipment are stated at cost, net of accumulated depreciation. Depreciation expense is recorded on a straight-line basis over the estimated useful life of the asset. Construction in progress reflects amounts incurred for construction or improvements of property, plant or equipment that have not been placed in service. Costs of construction of certain long-lived assets include capitalized interest, which is amortized over the estimated useful life of the related asset. The cost and accumulated depreciation of assets retired or sold are removed from the respective asset category, and any gain or loss is recognized in our consolidated statements of operations and comprehensive income (loss).
Leases	Leases We determine if an arrangement is a lease at contract inception based on the facts and circumstances present in the arrangement. All of our leases are classified as operating leases. Operating lease right-of-use assets represent our right to use an underlying asset for the lease term and operating lease liabilities represent our obligation to make lease payments arising from the leasing arrangement. Operating lease liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of our leases do not provide an implicit rate, in determining the operating lease liabilities, we use an estimate of our incremental borrowing rate based on the information available at commencement. Operating lease right-of-use assets are measured as the lease liability plus initial direct costs and prepaid lease payments less lease incentives. Single lease cost is recognized on a straight-line basis over the lease term. We do not separate non-lease components from lease components for all classes of underlying assets. Leases with an initial term of 12 months or less are not recorded on the consolidated balance sheets, and the respective lease cost is recognized in the consolidated statements of operations and comprehensive income (loss) on a straight-line basis over the term of the lease.
Clinical Accruals	Clinical Accruals We record accrued liabilities related to products we have received or services that we have incurred, specifically related to ongoing preclinical studies and clinical trials, for which service providers have not yet billed us, or when billing terms under these contracts do not coincide with the timing of when the work is performed, on our consolidated balance sheets. These costs primarily relate to third-party clinical management costs, laboratory and analysis costs, toxicology studies and investigator fees. The assessment of these costs requires judgment based on our knowledge of the research and development programs, services performed for the period, experience with related activities and the expected duration of the third-party service contract, where applicable.
Revenue Recognition	Revenue Recognition We recognize revenue when control of promised goods or services is transferred to a customer at an amount that reflects the consideration to which we expect to be entitled in exchange for those goods or services. To determine revenue recognition, we perform the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligation(s) in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligation(s) in the contract; and (v) recognize revenue when (or as) we satisfy the performance obligation(s). We only apply the five-step model to contracts when collectability of the consideration to which we are entitled in exchange for the goods or services we transfer to the customer is determined to be probable. At contract inception, once the contract is determined to be within the scope of Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers, or ASC 606, we assess whether the goods or services promised within each contract are distinct and, therefore, represent a separate performance obligation. Goods and services that are determined not to be distinct are combined with other promised goods and services until a distinct bundle is identified. We then allocate the transaction price (the amount of consideration we expect to be entitled to from a customer in exchange for the promised goods or services) to each performance obligation and recognize the associated revenue when (or as) each performance obligation is satisfied. Our estimate of the transaction price for each contract includes all variable consideration to which we expect to be entitled. Amounts are recorded as accounts receivable when our right to consideration is unconditional. We do not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. We expense incremental costs of obtaining a contract as and when incurred if the expected amortization period of the asset that we would have recognized is one year or less or the amount is immaterial. As of December 31, 2025 and 2024, we had not capitalized any costs to obtain any of our contracts. Net Product Revenues Our net product revenues are recognized, net of variable consideration related to certain allowances and accruals, at the time the customer obtains control of our product. We use the expected value method, which is the sum of probability-weighted amounts in a range of possible consideration amounts, or the most likely amount method, which is the single most likely amount in a range of possible considerations, to estimate variable consideration related to our product sales. We use the expected value method to estimate variable consideration for certain rebates, chargebacks, product returns, and other incentives and we use the most likely amount method for certain rebates, trade discounts and allowances. We record reserves, based on contractual terms, for components related to product sold during the reporting period, as well as our estimate of product that remains in the distribution channel inventory at the end of the reporting period that we expect will be sold to qualified healthcare providers. On a quarterly basis, we update our estimates and record any needed adjustments in the period we identify the adjustments. The following are the components of variable consideration related to product revenues: Chargebacks: We estimate obligations resulting from contractual commitments with the government and other entities to sell products to qualified healthcare providers at prices lower than the list prices charged to the customer who directly purchases from us. The customer charges us for the difference between what it pays to us for the product and the selling price to the qualified healthcare providers. We estimate chargebacks that we expect to pay and deduct the chargeback amount in full from gross product revenues and accounts receivable at the time we recognize the related revenues. Rebates: We are subject to discount obligations under government programs, including Medicare and Medicaid in the U.S. and similar programs in certain other countries, including countries in which we are accruing for estimated rebates because final pricing has not yet been negotiated. We are also subject to potential rebates in connection with our value-based agreements with certain commercial payors. We record reserves for rebates in the same period the related product revenue is recognized, resulting in a reduction of product revenues and a current liability that is included in accrued expenses on our consolidated balance sheet. Our estimate for rebates is based on statutory discount rates, expected utilization or an estimated number of patients on treatment, as applicable. Other incentives and allowances: Other incentives and allowances include trade discounts and allowances, fees, product returns, and co-payment assistance that we provide to patients with commercial insurance that have coverage and reside in states that allow co-payment assistance. We estimate discounts and fees based on contract terms and other relevant factors, and deduct these discounts and fees in full from gross product revenues and accounts receivable at the time we recognize the related revenues. We estimate the amount of product that will be returned based on our sales history for product that is damaged, defective or expired in accordance with the terms of each customer agreement, and we estimate the average co-payment assistance amounts for our products based on expected customer demographics. We record any such amounts within accrued expenses or product accounts receivable on our consolidated balance sheets. Net Revenues from Collaborations We earn revenue in connection with collaboration agreements that allow our collaborators to utilize our technology platforms and develop product candidates. Our significant collaboration agreements are detailed in Note 4, Net Revenues from Collaborations. For each collaborator, we discuss our revenue recognition, including our significant performance obligations under each agreement. At contract inception, we assess whether the collaboration arrangements are within the scope of ASC 606 or ASC Topic 808, Collaborative Arrangements, or ASC 808, to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed based on the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, we first determine which elements of the arrangement are within the scope of ASC 808 and which elements are within the scope of ASC 606. For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, either by analogy to authoritative accounting literature or by applying a reasonable and rational policy election. For elements of collaboration arrangements that are accounted for pursuant to ASC 606, we identify the performance obligations and allocate the total consideration we expect to receive on a relative standalone selling price basis to each performance obligation. Variable consideration, such as performance-based milestones, will be included in the total consideration if we expect to receive such consideration and if it is probable that the inclusion of the variable consideration will not result in a significant reversal in the cumulative amount of revenue recognized under the arrangement. Our estimate of the total consideration we expect to receive under each collaboration arrangement is updated for each reporting period, and any adjustments to revenue are recorded on a cumulative catch-up basis. We exclude sales-based royalty and milestone payments from the total consideration we expect to receive until the underlying sales occur because the license to our intellectual property is deemed to be the predominant item to which the royalties or milestones relate as it is the primary driver of value in our collaboration arrangements. Key assumptions to determine the standalone selling price may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. We recognize revenue associated with each performance obligation as the control over the promised goods or services transfer to our collaborator which occurs either at a point in time or over time. If control transfers over time, revenue is recognized by using a method of measuring progress that best depicts the transfer of goods or services. We evaluate the measure of progress and related inputs each reporting period and any resulting adjustments to revenue are recorded on a cumulative catch-up basis. Consideration received that does not meet the requirements to satisfy ASC 808 or ASC 606 revenue recognition criteria is recorded as deferred revenue in the accompanying consolidated balance sheets, classified as either short-term (less than 12 months) or long-term (more than 12 months) deferred revenue based on our best estimate of when such revenue will be recognized.
Cost of Goods Sold	Cost of Goods Sold Cost of goods sold includes the cost of producing and distributing inventories that are related to product revenues during the respective period (including salaries and related costs, stock-based compensation expenses and benefits for employees involved with production and distribution, supplies, external services, freight and indirect overhead costs), third-party royalties payable on our net product revenues and amortization of intangible assets associated with the sale of our products. Cost of goods sold may also include costs related to excess or obsolete inventory adjustment charges, abnormal costs, unabsorbed manufacturing and overhead costs, and manufacturing variances.
Cost of Collaborations and Royalties	Cost of Collaborations and Royalties Cost of collaborations and royalties includes costs we incur in connection with providing commercial drug supplies, such as GalNAc material, to collaborators, in addition to royalties we owe to third parties on the net sales of licensed products.
Income Taxes	Income Taxes We account for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between financial statement carrying amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Valuation allowances are provided if, based on the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. Uncertain tax positions, for which management’s assessment is that there is a more than 50% probability of sustaining the position upon challenge by a taxing authority based upon its technical merits, are subject to certain recognition and measurement criteria. The nature of the uncertain tax positions is often very complex and subject to change, and the amounts at issue can be substantial. We develop our cumulative probability assessment of the measurement of uncertain tax positions using internal experience, judgment and assistance from professional advisors. We re-evaluate these uncertain tax positions on a quarterly basis based on a number of factors including, but not limited to, changes in facts or circumstances, changes in tax law, and effectively settled issues under audit and new audit activity. Any change in these factors could result in the recognition of a tax benefit or an additional charge to the tax provision.
Research and Development Expenses	Research and Development Expenses We record research and development expenses as incurred. Included in research and development expenses are salaries and related costs, stock-based compensation expenses, benefits and other operating costs, facilities, supplies, external services, clinical trial and manufacturing costs, certain costs related to our collaboration arrangements, and overhead directly related to our research and development operations, as well as costs to acquire technology licenses. We have entered into several license agreements for rights to utilize certain technologies. The terms of the licenses may provide for upfront payments, annual maintenance payments, milestone payments based upon certain specified events being achieved and royalties on product sales. We charge costs to acquire and maintain licensed technology that has not reached technological feasibility and does not have alternative future use to research and development expenses as incurred.
Stock-Based Compensation	Stock-Based Compensation We recognize stock-based compensation expense for grants under our stock incentive plans and employee stock purchase plan. We account for all stock-based awards granted to employees at their fair value and recognize compensation expense over each employee’s requisite service period, which is generally the vesting period of the award. Determining the amount of stock-based compensation to be recorded requires us to develop estimates of fair values of stock awards as of the grant date. We calculate the grant date fair values of stock options using the Black-Scholes valuation model, which requires us to make assumptions, including but not limited to expected stock price volatility over the term of the awards and the expected term of stock options. The fair value of restricted stock awards granted to employees is based upon the quoted closing market price per share on the date of grant. We have performance conditions included in certain of our restricted stock awards that are based upon the achievement of pre-specified clinical development, regulatory, commercial and/or financial performance events. As the outcome of each event has inherent risk and uncertainties, and a positive outcome may not be known until the event is achieved, we begin to recognize the grant-date fair value of the performance-based restricted stock awards when we determine the achievement of each performance condition is deemed probable, a determination which requires significant judgment by management. On the date the performance condition is deemed probable, we record a cumulative expense catch-up, with remaining expense amortized over the remaining service period. The number of shares that ultimately vests for certain performance-based restricted stock units may range from 0% to 200% of the target award, depending on the assessed level of achievement of the specified performance conditions. We reassess the probability of achievement at each reporting period and adjust cumulative expense as necessary.
Advertising Expenses	Advertising Expenses We expense the costs of advertising as incurred.
Liabilities Related to the Sale of Future Royalties and Development Funding	Liabilities Related to the Sale of Future Royalties and Development Funding When we enter into or modify an arrangement with a party to fund our research and development costs, we assess if the arrangement meets all of the characteristics of a derivative or contains an embedded derivative, and if so, whether any of the scope exceptions to derivative accounting are applicable. If the arrangement is not accounted for as a derivative, we assess if successful completion of the research and development is already probable at the time the funding is received, and if so we apply the guidance in ASC Topic 470-10, Debt - Overall, or ASC 470-10. If the research and development risk is substantive, such that it is not yet probable development will be successful, we apply the guidance in ASC Topic 730-20, Research and Development - Research and Development Arrangements, or ASC 730-20, to evaluate whether the research and development funding is a liability to repay the funding party or an obligation to perform contractual services. If the transfer of financial risk involved with the research and development is substantive and genuine, and we have no explicit or implicit obligation to repay any of the funds, we account for the arrangement as an obligation to perform contractual research and development services. If the transfer of financial risk is not substantive and genuine, we recognize a liability to repay the funding party. Due to the absence of specific guidance with respect to the subsequent accounting for funds recognized as repayment obligations in ASC 730-20, we apply the concepts of ASC 470-10 to account for the liability to repay the funding party when a repayment obligation has been incurred. We account for the liabilities related to the sale of future royalties and development funding as debt financings. Interest on these liabilities is recognized using the effective interest rate method over the repayment period. The liabilities related to the sale of future royalties and development funding and the related interest expense are based on our current estimates of future royalties and milestones expected to be paid and received over the life of the arrangement, which we determine by using third-party data to estimate Leqvio’s and AMVUTTRA’s global net revenues. At each balance sheet date, we assess the expected payments and we account for any change by prospectively adjusting the effective interest rate and related interest expense.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is comprised of net income (loss) and certain changes in stockholders’ equity (deficit) that are excluded from net income (loss). Other comprehensive income (loss) includes unrealized holding gains and losses on marketable debt securities classified as available-for-sale, foreign currency translation adjustments (if the functional currency is not the U.S. dollar), and certain changes in the fair value of the plan assets and projected benefit obligations attributed to our defined benefit pension plan.
Recently Adopted Accounting Pronouncements and Recent Accounting Pronouncements	Recently Adopted Accounting Pronouncements In December 2023, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2023-09, Improvements to Income Tax Disclosures, or ASU 2023-09, which requires entities to disclose disaggregated information about their effective tax rate reconciliation as well as expanded information on income taxes paid by jurisdiction. We adopted ASU 2023-09 in the fourth quarter of 2025 and applied it prospectively, as disclosed in Note 14, Income Taxes. The adoption did not have an impact on our consolidated financial statements. In November 2024, the FASB issued ASU 2024-04, Induced Conversions of Convertible Debt Instruments, or ASU 2024-04, which clarifies the requirements for determining whether certain settlements of convertible debt instruments should be accounted for as an induced conversion or extinguishment of convertible debt. We adopted ASU 2024-04 in the third quarter of 2025 and applied it on a retrospective basis as of January 1, 2025. The adoption did not have a material effect on our consolidated financial statements, but we applied the guidance to the repurchase of our 1.00% Convertible Senior Notes due 2027 in the third and fourth quarters of 2025. Please refer to Note 8, Convertible Debt and Other Financing, for further information related to the repurchase. In September 2025, the FASB issued ASU 2025-07, Derivatives Scope Refinements and Scope Clarification for Share-Based Noncash Consideration from a Customer in a Revenue Contract, or ASU 2025-07. The guidance refines the scope of ASC Topic 815, Derivatives and Hedging, or ASC 815, to clarify which contracts are subject to derivative accounting. The guidance also provides clarification under ASC 606 for share-based payments from a customer in a revenue contract. We adopted the standard in the fourth quarter of 2025 on a modified retrospective basis as of January 1, 2025. As a result of the new derivative scope exception created by the standard, we now account for the previous development derivative liability similar to a debt obligation under the guidance of ASC 470-10. Refer to Note 9, Liabilities Related To The Sale Of Future Royalties And Development Funding, for additional information related to the adoption of this new standard and the related effects to the consolidated financial statements. We elected to adopt the guidance of ASU 2025-07 because we believe the accounting treatment of this guidance better reflects the economics of our existing research and development funding arrangements and provides more decision-useful information when derivative accounting is not applied. The guidance of ASU 2025-07 related to share-based noncash consideration received in exchange for the transfer of goods or services to a customer did not have any impact on our consolidated financial statements upon adoption. Recent Accounting Pronouncements In September 2025, the FASB issued ASU 2025-06, Targeted Improvements to the Accounting for Internal-Use Software, which modernizes the guidance for internal-use software costs by increasing the operability of the recognition guidance considering different methods of software development. The standard will be effective for annual reporting periods beginning after December 15, 2027, as well as interim period reporting periods within those annual reporting periods, with early adoption permitted. The standard updates may be applied on a prospective, retrospective, or modified retrospective approach. We are currently evaluating the impact this guidance could have on our consolidated financial statements and related disclosures. In November 2024, the FASB issued ASU 2024-03, Disaggregation of Income Statement Expenses, or ASU 2024-03, which is intended to improve disclosures by requiring additional information about specific expense categories in the notes to the financial statements on an annual and interim basis. The standard will be effective for annual reporting periods beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027, with early adoption permitted. The standard updates may be applied on either a prospective or retrospective basis. We are currently evaluating the disclosure requirements related to ASU 2024-03.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

12 Months Ended

Dec. 31, 2025

Basis of Presentation and Principles of Consolidation

The accompanying consolidated financial statements reflect the operations of Alnylam and our wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated. Certain prior period amounts in the consolidated financial statements have been reclassified to conform to the current period presentation.

Use of Estimates

The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. In our consolidated financial statements, we use estimates and assumptions related to our inventory valuation and related reserves, clinical accruals, liabilities related to the sale of future royalties and development funding, income taxes, deferred tax asset valuation allowances, revenue recognition, research and development expenses, and stock-based compensation expense. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable. Actual results could differ from those estimates. Changes in estimates are reflected in reported results in the period in which they become known.

Liquidity

Based on our current operating plan, we believe that our cash, cash equivalents, marketable securities, as well as the revenue we expect to generate from product sales and under our existing collaborations, including royalties on sales of Leqvio and Qfitlia, and available borrowing capacity under the revolving credit agreement, or the Revolving Credit Agreement, as of December 31, 2025, will be sufficient to satisfy our near-term capital and operating needs for at least the next 12 months from the filing date of this Annual Report on Form 10-K.

Concentrations of Credit Risk and Significant Customers

Financial instruments that potentially expose us to concentrations of credit risk primarily consist of cash, cash equivalents and marketable securities. As of December 31, 2025 and 2024, substantially all of our cash, cash equivalents and marketable securities were invested in money market funds, certificates of deposit, commercial paper, corporate notes, U.S. government-sponsored enterprise securities and U.S. treasury securities through highly rated financial institutions. Corporate notes may also include foreign bonds denominated in U.S. dollars. Investments are restricted, in accordance with our investment policy, to a concentration limit per issuer.

Fair Value Measurements

The fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair values determined by Level 2 inputs utilize data points that are observable, such as quoted prices (adjusted), interest rates and yield curves. Fair values determined by Level 3 inputs utilize unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. The fair value hierarchy level is determined by the lowest level of significant input.

Investments in Marketable Securities and Cash Equivalents

We invest our excess cash balances in marketable debt securities and classify our investments as either trading, held-to-maturity or available-for-sale based on facts and circumstances present at the time we purchased the securities. As of December 31, 2025 and 2024, we classified all of our investments in debt securities as available-for-sale and as current assets on the consolidated balance sheets as they represent the investment of funds available for current operations. We report available-for-sale debt securities at fair value at each balance sheet date, for which fair value measurement data is obtained from independent pricing services, and include any unrealized holding gains and losses (the adjustment to fair value) in accumulated other comprehensive loss. Realized gains and losses are determined using the specific identification method and are included in other income (expense), net. If any adjustment to fair value reflects a decline in the value of the marketable debt securities, we consider all available evidence to evaluate if an impairment loss exists, and if so, mark the investment to market through a charge to our consolidated statements of operations and comprehensive income (loss). We did not record any impairment charges related to our marketable debt securities during the years ended December 31, 2025, 2024 or 2023. Our marketable debt securities are classified as cash equivalents if the original maturity, from the date of purchase, is 90 days or less, and as marketable debt securities if the original maturity, from the date of purchase, is in excess of 90 days. Our cash equivalents are generally composed of commercial paper, corporate notes, U.S. government-sponsored enterprise securities, U.S. treasury securities, money market funds and certificates of deposit.

We measure marketable equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of an investee), which have readily available prices, at fair value with changes in fair value recognized in other income (expense), net on our consolidated statements of operations and comprehensive income (loss).

Accounts Receivable, Net

We record accounts receivable net of prompt payment discounts and chargebacks based on contractual terms. As of December 31, 2025 and 2024, based on our estimation of expected write-offs, we determined an allowance for doubtful accounts was not material. We have standard payment terms that generally require payment within approximately 30 to 90 days. Accounts receivable, net on our consolidated balance sheets also includes collaboration receivables and royalty receivables.

Inventory

Inventory is measured at the lower of cost or estimated net realizable value and classified based on the anticipation of when it will be consumed either within our normal operating cycle (short-term) or beyond (long-term). We use a standard cost basis, which approximates cost determined on a first-in, first-out basis. Inventory costs include all raw materials, direct conversion

costs and overhead. Raw and intermediate materials that may be used for either research and development or commercial purposes are classified as inventory until the material is consumed or otherwise allocated for research and development. If the material is used for research and development, it is recorded as research and development expenses once that determination is made.

We capitalize inventory costs that are expected to be sold commercially once we determine it is probable that the inventory costs will be recovered through commercial sale based on the review of several factors, including (i) the likelihood that all required regulatory approvals will be received, considering any special filing status, (ii) the expected timing of validation (if not yet completed) of manufacturing processes in the associated facility, (iii) the expected expiration of the inventory, (iv) logistical or commercial constraints that may impede the timely distribution and sale of the product, including transport requirements and reimbursement status, (v) current market factors, including competitive landscape and pricing, (vi) threatened or anticipated litigation challenges, (vii) history of approvals of similar products or formulations, and (viii) U.S. Food and Drug Administration, or FDA, (or other appropriate regulatory agencies) correspondence regarding the safety and efficacy of the product. Prior to the capitalization of inventory costs, we record such costs as research and development expenses on our consolidated statements of operations and comprehensive income (loss).

We reduce our inventory to net realizable value for potentially excess, dated or obsolete inventory based on our quarterly assessment of the recoverability of our capitalized inventory. We periodically review inventory levels to identify what may expire prior to expected sale or has a cost basis in excess of its estimated realizable value and write down such inventories as appropriate.

Property, Plant and Equipment, Net

Property, plant and equipment are stated at cost, net of accumulated depreciation. Depreciation expense is recorded on a straight-line basis over the estimated useful life of the asset. Construction in progress reflects amounts incurred for construction or improvements of property, plant or equipment that have not been placed in service. Costs of construction of certain long-lived assets include capitalized interest, which is amortized over the estimated useful life of the related asset. The cost and accumulated depreciation of assets retired or sold are removed from the respective asset category, and any gain or loss is recognized in our consolidated statements of operations and comprehensive income (loss).

Leases

We determine if an arrangement is a lease at contract inception based on the facts and circumstances present in the arrangement. All of our leases are classified as operating leases. Operating lease right-of-use assets represent our right to use an underlying asset for the lease term and operating lease liabilities represent our obligation to make lease payments arising from the leasing arrangement. Operating lease liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of our leases do not provide an implicit rate, in determining the operating lease liabilities, we use an estimate of our incremental borrowing rate based on the information available at commencement. Operating lease right-of-use assets are measured as the lease liability plus initial direct costs and prepaid lease payments less lease incentives. Single lease cost is recognized on a straight-line basis over the lease term.

We do not separate non-lease components from lease components for all classes of underlying assets. Leases with an initial term of 12 months or less are not recorded on the consolidated balance sheets, and the respective lease cost is recognized in the consolidated statements of operations and comprehensive income (loss) on a straight-line basis over the term of the lease.

Clinical Accruals

We record accrued liabilities related to products we have received or services that we have incurred, specifically related to ongoing preclinical studies and clinical trials, for which service providers have not yet billed us, or when billing terms under these contracts do not coincide with the timing of when the work is performed, on our consolidated balance sheets. These costs

primarily relate to third-party clinical management costs, laboratory and analysis costs, toxicology studies and investigator fees. The assessment of these costs requires judgment based on our knowledge of the research and development programs, services performed for the period, experience with related activities and the expected duration of the third-party service contract, where applicable.

Revenue Recognition

We recognize revenue when control of promised goods or services is transferred to a customer at an amount that reflects the consideration to which we expect to be entitled in exchange for those goods or services. To determine revenue recognition, we perform the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligation(s) in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligation(s) in the contract; and (v) recognize revenue when (or as) we satisfy the performance obligation(s). We only apply the five-step model to contracts when collectability of the consideration to which we are entitled in exchange for the goods or services we transfer to the customer is determined to be probable.

At contract inception, once the contract is determined to be within the scope of Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers, or ASC 606, we assess whether the goods or services promised within each contract are distinct and, therefore, represent a separate performance obligation. Goods and services that are determined not to be distinct are combined with other promised goods and services until a distinct bundle is identified. We then allocate the transaction price (the amount of consideration we expect to be entitled to from a customer in exchange for the promised goods or services) to each performance obligation and recognize the associated revenue when (or as) each performance obligation is satisfied. Our estimate of the transaction price for each contract includes all variable consideration to which we expect to be entitled.

Amounts are recorded as accounts receivable when our right to consideration is unconditional. We do not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. We expense incremental costs of obtaining a contract as and when incurred if the expected amortization period of the asset that we would have recognized is one year or less or the amount is immaterial. As of December 31, 2025 and 2024, we had not capitalized any costs to obtain any of our contracts.

Net Product Revenues

Our net product revenues are recognized, net of variable consideration related to certain allowances and accruals, at the time the customer obtains control of our product. We use the expected value method, which is the sum of probability-weighted amounts in a range of possible consideration amounts, or the most likely amount method, which is the single most likely amount in a range of possible considerations, to estimate variable consideration related to our product sales. We use the expected value method to estimate variable consideration for certain rebates, chargebacks, product returns, and other incentives and we use the most likely amount method for certain rebates, trade discounts and allowances.

We record reserves, based on contractual terms, for components related to product sold during the reporting period, as well as our estimate of product that remains in the distribution channel inventory at the end of the reporting period that we expect will be sold to qualified healthcare providers. On a quarterly basis, we update our estimates and record any needed adjustments in the period we identify the adjustments. The following are the components of variable consideration related to product revenues:

Chargebacks: We estimate obligations resulting from contractual commitments with the government and other entities to sell products to qualified healthcare providers at prices lower than the list prices charged to the customer who directly purchases from us. The customer charges us for the difference between what it pays to us for the product and the selling price to the qualified healthcare providers. We estimate chargebacks that we expect to pay and deduct the chargeback amount in full from gross product revenues and accounts receivable at the time we recognize the related revenues.

Rebates: We are subject to discount obligations under government programs, including Medicare and Medicaid in the U.S. and similar programs in certain other countries, including countries in which we are accruing for estimated rebates because final pricing has not yet been negotiated. We are also subject to potential rebates in connection with our value-based agreements with certain commercial payors. We record reserves for rebates in the same period the related product revenue is recognized, resulting in a reduction of product revenues and a current liability that is included in accrued expenses on our consolidated balance sheet. Our estimate for rebates is based on statutory discount rates, expected utilization or an estimated number of patients on treatment, as applicable.

Other incentives and allowances: Other incentives and allowances include trade discounts and allowances, fees, product returns, and co-payment assistance that we provide to patients with commercial insurance that have coverage and reside in states that allow co-payment assistance. We estimate discounts and fees based on contract terms and other relevant factors, and deduct these discounts and fees in full from gross product revenues and accounts receivable at the time we recognize the related revenues. We estimate the amount of product that will be returned based on our sales history for product that is damaged,

defective or expired in accordance with the terms of each customer agreement, and we estimate the average co-payment assistance amounts for our products based on expected customer demographics. We record any such amounts within accrued expenses or product accounts receivable on our consolidated balance sheets.

Net Revenues from Collaborations

We earn revenue in connection with collaboration agreements that allow our collaborators to utilize our technology platforms and develop product candidates. Our significant collaboration agreements are detailed in Note 4, Net Revenues from Collaborations. For each collaborator, we discuss our revenue recognition, including our significant performance obligations under each agreement.

At contract inception, we assess whether the collaboration arrangements are within the scope of ASC 606 or ASC Topic 808, Collaborative Arrangements, or ASC 808, to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed based on the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, we first determine which elements of the arrangement are within the scope of ASC 808 and which elements are within the scope of ASC 606. For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, either by analogy to authoritative accounting literature or by applying a reasonable and rational policy election.

For elements of collaboration arrangements that are accounted for pursuant to ASC 606, we identify the performance obligations and allocate the total consideration we expect to receive on a relative standalone selling price basis to each performance obligation. Variable consideration, such as performance-based milestones, will be included in the total consideration if we expect to receive such consideration and if it is probable that the inclusion of the variable consideration will not result in a significant reversal in the cumulative amount of revenue recognized under the arrangement. Our estimate of the total consideration we expect to receive under each collaboration arrangement is updated for each reporting period, and any adjustments to revenue are recorded on a cumulative catch-up basis. We exclude sales-based royalty and milestone payments from the total consideration we expect to receive until the underlying sales occur because the license to our intellectual property is deemed to be the predominant item to which the royalties or milestones relate as it is the primary driver of value in our collaboration arrangements.

Key assumptions to determine the standalone selling price may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. We recognize revenue associated with each performance obligation as the control over the promised goods or services transfer to our collaborator which occurs either at a point in time or over time. If control transfers over time, revenue is recognized by using a method of measuring progress that best depicts the transfer of goods or services. We evaluate the measure of progress and related inputs each reporting period and any resulting adjustments to revenue are recorded on a cumulative catch-up basis.

Consideration received that does not meet the requirements to satisfy ASC 808 or ASC 606 revenue recognition criteria is recorded as deferred revenue in the accompanying consolidated balance sheets, classified as either short-term (less than 12 months) or long-term (more than 12 months) deferred revenue based on our best estimate of when such revenue will be recognized.

Cost of Goods Sold

Cost of goods sold includes the cost of producing and distributing inventories that are related to product revenues during the respective period (including salaries and related costs, stock-based compensation expenses and benefits for employees involved with production and distribution, supplies, external services, freight and indirect overhead costs), third-party royalties payable on our net product revenues and amortization of intangible assets associated with the sale of our products. Cost of goods sold may also include costs related to excess or obsolete inventory adjustment charges, abnormal costs, unabsorbed manufacturing and overhead costs, and manufacturing variances.

Cost of Collaborations and Royalties

Cost of collaborations and royalties includes costs we incur in connection with providing commercial drug supplies, such as GalNAc material, to collaborators, in addition to royalties we owe to third parties on the net sales of licensed products.

Income Taxes

We account for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between financial statement carrying amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Valuation allowances are provided if, based on the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

Uncertain tax positions, for which management’s assessment is that there is a more than 50% probability of sustaining the position upon challenge by a taxing authority based upon its technical merits, are subject to certain recognition and measurement criteria. The nature of the uncertain tax positions is often very complex and subject to change, and the amounts at issue can be substantial. We develop our cumulative probability assessment of the measurement of uncertain tax positions using internal experience, judgment and assistance from professional advisors. We re-evaluate these uncertain tax positions on a quarterly basis based on a number of factors including, but not limited to, changes in facts or circumstances, changes in tax law, and effectively settled issues under audit and new audit activity. Any change in these factors could result in the recognition of a tax benefit or an additional charge to the tax provision.

Research and Development Expenses

We record research and development expenses as incurred. Included in research and development expenses are salaries and related costs, stock-based compensation expenses, benefits and other operating costs, facilities, supplies, external services, clinical trial and manufacturing costs, certain costs related to our collaboration arrangements, and overhead directly related to our research and development operations, as well as costs to acquire technology licenses.

We have entered into several license agreements for rights to utilize certain technologies. The terms of the licenses may provide for upfront payments, annual maintenance payments, milestone payments based upon certain specified events being achieved and royalties on product sales. We charge costs to acquire and maintain licensed technology that has not reached technological feasibility and does not have alternative future use to research and development expenses as incurred.

Stock-Based Compensation

We recognize stock-based compensation expense for grants under our stock incentive plans and employee stock purchase plan. We account for all stock-based awards granted to employees at their fair value and recognize compensation expense over each employee’s requisite service period, which is generally the vesting period of the award. Determining the amount of stock-based compensation to be recorded requires us to develop estimates of fair values of stock awards as of the grant date. We calculate the grant date fair values of stock options using the Black-Scholes valuation model, which requires us to make assumptions, including but not limited to expected stock price volatility over the term of the awards and the expected term of stock options. The fair value of restricted stock awards granted to employees is based upon the quoted closing market price per share on the date of grant.

We have performance conditions included in certain of our restricted stock awards that are based upon the achievement of pre-specified clinical development, regulatory, commercial and/or financial performance events. As the outcome of each event has inherent risk and uncertainties, and a positive outcome may not be known until the event is achieved, we begin to recognize the grant-date fair value of the performance-based restricted stock awards when we determine the achievement of each performance condition is deemed probable, a determination which requires significant judgment by management. On the date the performance condition is deemed probable, we record a cumulative expense catch-up, with remaining expense amortized over the remaining service period. The number of shares that ultimately vests for certain performance-based restricted stock units may range from 0% to 200% of the target award, depending on the assessed level of achievement of the specified performance conditions. We reassess the probability of achievement at each reporting period and adjust cumulative expense as necessary.

Advertising Expenses

We expense the costs of advertising as incurred.

When we enter into or modify an arrangement with a party to fund our research and development costs, we assess if the arrangement meets all of the characteristics of a derivative or contains an embedded derivative, and if so, whether any of the scope exceptions to derivative accounting are applicable. If the arrangement is not accounted for as a derivative, we assess if successful completion of the research and development is already probable at the time the funding is received, and if so we apply the guidance in ASC Topic 470-10, Debt - Overall, or ASC 470-10. If the research and development risk is substantive, such that it is not yet probable development will be successful, we apply the guidance in ASC Topic 730-20, Research and Development - Research and Development Arrangements, or ASC 730-20, to evaluate whether the research and development funding is a liability to repay the funding party or an obligation to perform contractual services. If the transfer of financial risk involved with the research and development is substantive and genuine, and we have no explicit or implicit obligation to repay any of the funds, we account for the arrangement as an obligation to perform contractual research and development services. If the transfer of financial risk is not substantive and genuine, we recognize a liability to repay the funding party. Due to the absence of specific guidance with respect to the subsequent accounting for funds recognized as repayment obligations in ASC 730-20, we apply the concepts of ASC 470-10 to account for the liability to repay the funding party when a repayment obligation has been incurred.

We account for the liabilities related to the sale of future royalties and development funding as debt financings. Interest on these liabilities is recognized using the effective interest rate method over the repayment period.

The liabilities related to the sale of future royalties and development funding and the related interest expense are based on our current estimates of future royalties and milestones expected to be paid and received over the life of the arrangement, which we determine by using third-party data to estimate Leqvio’s and AMVUTTRA’s global net revenues. At each balance sheet date, we assess the expected payments and we account for any change by prospectively adjusting the effective interest rate and related interest expense.

Comprehensive Income (Loss)

Comprehensive income (loss) is comprised of net income (loss) and certain changes in stockholders’ equity (deficit) that are excluded from net income (loss). Other comprehensive income (loss) includes unrealized holding gains and losses on marketable debt securities classified as available-for-sale, foreign currency translation adjustments (if the functional currency is not the U.S. dollar), and certain changes in the fair value of the plan assets and projected benefit obligations attributed to our defined benefit pension plan.

Recently Adopted Accounting Pronouncements and Recent Accounting Pronouncements

Recently Adopted Accounting Pronouncements

In December 2023, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2023-09, Improvements to Income Tax Disclosures, or ASU 2023-09, which requires entities to disclose disaggregated information about their effective tax rate reconciliation as well as expanded information on income taxes paid by jurisdiction. We adopted ASU 2023-09 in the fourth quarter of 2025 and applied it prospectively, as disclosed in Note 14, Income Taxes. The adoption did not have an impact on our consolidated financial statements.

In November 2024, the FASB issued ASU 2024-04, Induced Conversions of Convertible Debt Instruments, or ASU 2024-04, which clarifies the requirements for determining whether certain settlements of convertible debt instruments should be accounted for as an induced conversion or extinguishment of convertible debt. We adopted ASU 2024-04 in the third quarter of 2025 and applied it on a retrospective basis as of January 1, 2025. The adoption did not have a material effect on our consolidated financial statements, but we applied the guidance to the repurchase of our 1.00% Convertible Senior Notes due 2027 in the third and fourth quarters of 2025. Please refer to Note 8, Convertible Debt and Other Financing, for further information related to the repurchase.

In September 2025, the FASB issued ASU 2025-07, Derivatives Scope Refinements and Scope Clarification for Share-Based Noncash Consideration from a Customer in a Revenue Contract, or ASU 2025-07. The guidance refines the scope of ASC Topic 815, Derivatives and Hedging, or ASC 815, to clarify which contracts are subject to derivative accounting. The guidance also provides clarification under ASC 606 for share-based payments from a customer in a revenue contract. We adopted the standard in the fourth quarter of 2025 on a modified retrospective basis as of January 1, 2025. As a result of the new derivative scope exception created by the standard, we now account for the previous development derivative liability similar to a debt obligation under the guidance of ASC 470-10. Refer to Note 9, Liabilities Related To The Sale Of Future Royalties And Development Funding, for additional information related to the adoption of this new standard and the related effects to the consolidated financial statements. We elected to adopt the guidance of ASU 2025-07 because we believe the accounting treatment of this guidance better reflects the economics of our existing research and development funding arrangements and provides more decision-useful information when derivative accounting is not applied. The guidance of ASU 2025-07 related to share-based noncash consideration received in exchange for the transfer of goods or services to a customer did not have any impact on our consolidated financial statements upon adoption.

Recent Accounting Pronouncements

In September 2025, the FASB issued ASU 2025-06, Targeted Improvements to the Accounting for Internal-Use Software, which modernizes the guidance for internal-use software costs by increasing the operability of the recognition guidance considering different methods of software development. The standard will be effective for annual reporting periods beginning after December 15, 2027, as well as interim period reporting periods within those annual reporting periods, with early adoption permitted. The standard updates may be applied on a prospective, retrospective, or modified retrospective approach. We are currently evaluating the impact this guidance could have on our consolidated financial statements and related disclosures.

In November 2024, the FASB issued ASU 2024-03, Disaggregation of Income Statement Expenses, or ASU 2024-03, which is intended to improve disclosures by requiring additional information about specific expense categories in the notes to the financial statements on an annual and interim basis. The standard will be effective for annual reporting periods beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027, with early adoption permitted. The standard updates may be applied on either a prospective or retrospective basis. We are currently evaluating the disclosure requirements related to ASU 2024-03.