HER2 mutations are targetable alterations in patients with hormone-receptor positive (HR+) metastatic breast cancer (MBC). In the SUMMIT basket study, patients with HER2-mutant MBC received neratinib monotherapy, neratinib+fulvestrant, or neratinib+fulvestrant+trastuzumab (N+F+T). We report results from 71 patients with HR+, HER2-mutant MBC, including 21 (7 in each arm) from a randomized substudy of fulvestrant versus fulvestrant+trastuzumab (F+T) versus N+F+T.