This phase 1 study (NCT05463952) investigated safety, pharmacokinetics, and antitumor activity of vepdegestrant in Japanese patients with ER-positive (ER+)/human epidermal growth factor receptor 2–negative (HER2-) advanced breast cancer at the 200-mg once daily (QD) recommended phase 3 dose. The gene discussed is ERBB2; the disease is breast cancer.