X0306 4 2021-12-08 0 0001840233 Xilio Therapeutics, Inc. XLO 0001703031 Bain Capital Life Sciences Investors, LLC 200 CLARENDON STREET BOSTON MA 02116 0 0 1 0 0001773187 Bain Capital Life Sciences Fund II, L.P. 200 CLARENDON STREET BOSTON MA 02116 0 0 1 0 0001796155 Bain Capital Life Sciences Investors II, LLC 200 CLARENDON STREET BOSTON MA 02116 0 0 1 0 0001818893 BCLS II Investco, LP 200 CLARENDON STREET BOSTON MA 02116 0 0 1 0 0001819101 BCLS II Investo (GP), LLC 200 CLARENDON STREET BOSTON MA 02116 0 0 1 0 0001702921 BCIP Life Sciences Associates, LP 200 CLARENDON STREET BOSTON MA 02116 0 0 1 0 Common Stock 2021-12-08 4 P 0 13458 9.6206 A 2776402 I See footnotes Common Stock 2021-12-09 4 P 0 4011 9.8442 A 2780413 I See footnotes Common Stock 2021-12-10 4 P 0 21812 9.5619 A 2802225 I See footnotes Common Stock 2021-12-10 4 P 0 3188 10.2378 A 2805413 I See footnotes On December 8, 2021, December 9, 2021 and December 10, 2021, BCLS II Investco, LP ("BCLS II Investco") purchased 13,458, 4,011 and 25,000 shares of the Issuer's common stock, respectively. The price reported in Column 4 is a weighted average price. The shares were purchased in multiple transactions at prices ranging from $9.41 to $9.75, inclusive. The reporting persons undertake to provide to the Issuer, any securityholder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the ranges set forth in footnotes (2) through (5) to this Form 4. The price reported in Column 4 is a weighted average price. The shares were purchased in multiple transactions at prices ranging from $9.53 to $10.00, inclusive. The price reported in Column 4 is a weighted average price. The shares were purchased in multiple transactions at prices ranging from $9.07 to $10.00, inclusive. The price reported in Column 4 is a weighted average price. The shares were purchased in multiple transactions at prices ranging from $10.13 to $10.25, inclusive. Represents shares of the Issuer's common stock held by Bain Capital Life Sciences Fund II, L.P. ("BCLS II"), BCLS II Investco and BCIP Life Sciences Associates, LP ("BCIPLS" and, together with BCLS II and BCLS II Investco, the "Bain Capital Life Sciences Entities"). Following the completion of the transactions reported in footnote (1) to this Form 4, BCLS II, BCLS II Investco and BCIPLS each holds 1,348,682, 1,292,469 and 164,262 shares of the Issuer's common stock, respectively. Bain Capital Life Sciences Investors II, LLC ("BCLSI II") is the general partner of BCLS II. As a result, BCLSI II may be deemed to share voting and dispositive power with respect to the securities held by BCLS II. BCLSI II disclaims beneficial ownership of such securities except to the extent of its pecuniary interest therein. BCLS II Investco (GP), LLC ("BCLS II Investco GP"), whose manager is BCLS II, is the general partner of BCLS II Investco. As a result, each of BCLSI II, BCLS II and BCLS II Investco GP may be deemed to share voting and dispositive power with respect to the securities held by BCLS II Investco. Each of BCLSI II, BCLS II and BCLS II Investco GP disclaims beneficial ownership of such securities except to the extent of its pecuniary interest therein. Boylston Coinvestors, LLC is the general partner of BCIPLS. Bain Capital Life Sciences Investors, LLC ("BCLSI") is the manager of BCLSI II and governs the investment strategy and decision-making process with respect to investments held by BCIPLS. As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Bain Capital Life Sciences Entities. BCLSI disclaims beneficial ownership of such securities except to the extent of its pecuniary interest therein. Exhibit 99.1: Joint Filer Information See Signatures Included in Exhibit 99.1 2021-12-10