Deferred revenue by revenue classification as of December 31, 2020 and 2019 was as follows (in thousands):

	December 31,
	2020		2019
Current deferred revenue
Sequencing	$	15,463	$	12,482
Development		57,856		48,512
Total current deferred revenue		73,319		60,994
Non-current deferred revenue
Sequencing		724		1,459
Development		162,894		217,873
Total non-current deferred revenue		163,618		219,332
Total current and non-current deferred revenue	$	236,937	$	280,326

Deferred revenue from our Genentech Agreement represents $55.1 million and $157.0 million of the current and non-current development deferred revenue balances, respectively, as of December 31, 2020 and $48.1 million and $216.8 million of the current and non-current development deferred revenue balances, respectively, as of December 31, 2019. 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 six to seven years. 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.

Changes in deferred revenue during the year ended December 31, 2020 were as follows (in thousands):

Deferred revenue balance at December 31, 2019	$	280,326
Additions to deferred revenue during the period		26,434
Revenue recognized during the period		(69,823	)
Deferred revenue balance at December 31, 2020	$	236,937

As of December 31, 2020, $61.6 million was recognized as revenue that was included in the deferred revenue balance at December 31, 2019. This is inclusive of $1.0 million of sequencing revenue recognized as a result of cancelled biopharmaceutical customer sequencing contracts and $0.6 million of sequencing revenue related to Medicare that was recognized due to our determination that additional testing for specific patients was remote.