During the three months ended March 31, 2024, we sold all of our available-for-sale debt securities and used the proceeds to partially fund our acquisition of CymaBay discussed in Note 6. Acquisitions, Collaborations and Other Arrangements. As such, there are no balances as of June 30, 2024 in the following tables.


	December 31, 2023
(in millions)		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
U.S. treasury securities		$	427	$	—	$	(1)	$	426
U.S. government agencies securities		127		—		—		127
Non-U.S. government securities		10		—		—		10
Certificates of deposit		45		—		—		45
Corporate debt securities		1,455		4		(8)		1,451
Residential mortgage and asset-backed securities		366		1		—		367
Total		$	2,430	$	5	$	(10)	$	2,426

The following table summarizes information related to available-for-sale debt securities that have been in a continuous unrealized loss position, classified by length of time, as of December 31, 2023:


	December 31, 2023
	Less Than 12 Months				12 Months or Longer				Total
(in millions)	Gross Unrealized Losses		Estimated Fair Value		Gross Unrealized Losses		Estimated Fair Value		Gross Unrealized Losses		Estimated Fair Value
U.S. treasury securities	$	—	$	161	$	(1)	$	48	$	(1)	$	209
U.S. government agencies securities	—		106		—		2		—		108
Non-U.S. government securities	—		5		—		5		—		10
Corporate debt securities	(1)		333		(7)		546		(8)		878
Residential mortgage and asset-backed securities	—		123		—		24		—		147
Total	$	(2)	$	727	$	(8)	$	624	$	(10)	$	1,351

The following table summarizes the classification of our available-for-sale debt securities in our Condensed Consolidated Balance Sheets as of December 31, 2023:


(in millions)	December 31, 2023
Cash and cash equivalents	$	83
Short-term marketable debt securities	1,179
Long-term marketable debt securities	1,163
Total	$	2,426


(in millions)	June 30, 2024		December 31, 2023
Equity securities measured at fair value:
Cash and cash equivalents	$	1,525	$	4,465
Prepaid and other current assets	1,426		1,086
Other long-term assets	330		656
Equity method investments and other equity investments without readily determinable fair values:
Other long-term assets	376		$	340
Total	$	3,657	$	6,547

For our equity method investments in Galapagos and Arcus Biosciences, Inc. (“Arcus”), we elected and applied the fair value option as we believe it best reflects the underlying economics of these investments. Our investment in Galapagos is subject to certain lock-up provisions until August 2024 and was classified in Prepaid and other current assets as of June 30, 2024 and December 31, 2023 at $417 million and $686 million, respectively. Our investment in Arcus was classified in Prepaid and other current assets as of June 30, 2024 and December 31, 2023 at $458 million and $283 million, respectively.

The following table summarizes net unrealized gains and losses on equity securities still held as of the respective balance sheet dates, included in Other (income) expense, net on our Condensed Consolidated Statements of Operations:


	Three Months Ended				Six Months Ended
	June 30,				June 30,
(in millions)	2024		2023		2024		2023
Unrealized loss (gain) from equity securities still held, net	$	392	$	(69)	$	412	$	121