Between April 2013 and March 2020, GDC-0810 was administered to 152 female patients who were postmenopausal with ER + (HER2 −) breast cancer, as a single agent (9 dose-escalation cohorts in phase Ia [n = 41] and 4 dose-expansion cohorts in phase IIa [n = 101]) and as combination therapy (2 dose-escalation cohorts in phase Ib [n = 10]). The gene discussed is ESR1; the disease is breast cancer.