Deferred revenue by revenue classification as of June 30, 2021 and December 31, 2020 was as follows (in thousands):

	June 30, 2021		December 31, 2020
Current deferred revenue
Sequencing	$	15,746	$	15,463
Development		67,807		57,856
Total current deferred revenue		83,553		73,319
Non-current deferred revenue
Sequencing		298		724
Development		119,344		162,894
Total non-current deferred revenue		119,642		163,618
Total current and non-current deferred revenue	$	203,195	$	236,937

Deferred revenue from our Genentech Agreement represents $64.8 million and $114.4 million of the current and non-current development deferred revenue balances, respectively, as of June 30, 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 June 30, 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 three and six months ended June 30, 2021, we recognized $2.6 million and $3.4 million in revenue, respectively, 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 $0.5 million and $0.7 million of sequencing revenue during the three and six months ended June 30, 2021, respectively, related to Medicare reimbursements resulting from our determination that the likelihood of additional testing for specific patients was remote.

Changes in deferred revenue during the six months ended June 30, 2021 were as follows (in thousands):

Deferred revenue balance at December 31, 2020	$	236,937
Additions to deferred revenue during the period		14,141
Revenue recognized during the period		(47,883	)
Deferred revenue balance at June 30, 2021	$	203,195

As of June 30, 2021, $40.6 million was recognized as revenue that was included in the deferred revenue balance at December 31, 2020.