X0306 4 2021-04-09 0 0001808865 iTeos Therapeutics, Inc. ITOS 0001609492 MPM BioVentures 2014, L.P. C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001134657 EVNIN LUKE C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001473930 Foley Todd C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001609493 MPM BioVentures 2014 (B), L.P. C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001609495 MPM Asset Management Investors BV2014 LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001765021 MPM BioVentures 2014 LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001765091 MPM BioVentures 2014 GP LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001729504 MPM BIOVENTURES 2018 (B), L.P. C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001734817 MPM ASSET MANAGEMENT INVESTORS BV2018 LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 Common Stock 2021-04-09 4 S 0 119 29.99 D 3665787 I See Footnote Transaction effected pursuant to a plan established pursuant to Rule 10b5-1. The shares were sold as follows: 71 by MPM BioVentures 2014, L.P. ("BV 2014"), 2 by MPM Asset Management Investors BV2014 LLC ("AM BV2014"), 45 by MPM BioVentures 2018, L.P. ("BV 2018") and 1 by MPM Asset Management Investors BV2018 LLC ("AM BV2018"). MPM BioVentures 2014 GP LLC and MPM BioVentures 2014 LLC ("BV 2014 LLC") are the direct and indirect general partners of BV 2014 and BV 2014(B). BV 2014 LLC is the manager of AM BV2014. MPM BioVentures 2018 GP LLC and MPM BioVentures 2018 LLC ("BV 2018 LLC") are the direct and indirect general partners of BV 2018 and BV 2018(B). BV 2018 LLC is the manager of AM BV2018. Messrs. Evnin and Foley are managing directors of BV 2014 LLC and BV 2018 LLC. The shares are held as follows: 2,068,870 by BV 2014, 128,561 by MPM BioVentures 2014(B), L.P. ("BV 2014(B)"), 71,210 by AM BV2014, 1,306,654 by BV 2018, 64,705 by MPM BioVentures 2018(B), L.P. ("BV 2018(B)") and 25,787 by AM BV2018. The Reporting Person disclaims beneficial ownership of the securities except to the extent of his pecuniary interest therein. See Form 4 for MPM BioVentures 2018, L.P for additional members of this joint filing. /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC, the managing member of MPM BioVentures 2014 GP LLC, the general partner of MPM BioVentures 2014, L.P. 2021-04-13 /s/ Luke Evnin 2021-04-13 /s/ Todd Foley 2021-04-13 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC, the managing member of MPM BioVentures 2014 GP LLC, the general partner of MPM BioVentures 2014 (B), L.P. 2021-04-13 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC, the manager of MPM Asset Management Investors BV2014 LLC 2021-04-13 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC 2021-04-13 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC, the managing member of MPM BioVentures 2014 GP LLC 2021-04-13 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2018 LLC, the managing member of MPM BioVentures 2018 GP LLC, the general partner of MPM BioVentures 2018 (B), L.P. 2021-04-13 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2018 LLC, the manager of MPM Asset Management Investors BV2018 LLC 2021-04-13