Deferred revenue by revenue classification as of December 31, 2021 and 2020 was as follows (in thousands):

	December 31,
	2021		2020
Current deferred revenue
Sequencing	$	21,915	$	15,463
Development		58,545		57,856
Total current deferred revenue		80,460		73,319
Non-current deferred revenue
Sequencing		174		724
Development		98,576		162,894
Total non-current deferred revenue		98,750		163,618
Total current and non-current deferred revenue	$	179,210	$	236,937

Deferred revenue from our Genentech Agreement represents $56.1 million and $94.0 million of the current and non-current development deferred revenue balances, respectively, as of December 31, 2021 and $55.1 million and $157.0 million of the current and non-current development deferred revenue balances, respectively, as of December 31, 2020. In general, we expect that the current amounts will be recognized as revenue within 12 months and the non-current amounts will be recognized as revenue over a period of approximately five to six years from December 31, 2021. This period of time represents an estimate of the research and development period to develop cellular therapies in oncology, which may be reduced or increased based on the various development activities.

During the year ended December 31, 2021, we recognized $3.6 million in revenue as a result of changes in estimates of total samples to be provided under certain of our agreements and cancelled biopharmaceutical customer sequencing contracts for which we had received upfront consideration. Additionally, we recognized $2.5 million of sequencing revenue during the year ended December 31, 2021 related to Medicare reimbursements resulting from our determination that the likelihood of additional testing for specific patients was remote and a change in our estimate of expected cumulative tests per patient for one of our covered indications.

Changes in deferred revenue during the year ended December 31, 2021 were as follows (in thousands):

Deferred revenue balance at December 31, 2020	$	236,937
Additions to deferred revenue during the period		38,062
Revenue recognized during the period		(95,789	)
Deferred revenue balance at December 31, 2021	$	179,210

As of December 31, 2021, $75.7 million was recognized as revenue that was included in the deferred revenue balance at December 31, 2020.