ERBB2 and breast cancer: By 2025, over 169 000 individuals in the United States are projected to be living with metastatic breast cancer (MBC).1 Hormone receptor (HR)-positive, human epidermal growth factor 2 (HER2)-negative (HR+/HER2−) breast cancer is the most common breast cancer subtype in the United States with an age-adjusted incidence rate of 87.2 cases per 100 000 women, per 2016-2020 data.2 The incidence of brain metastasis is 15% among patients with HR+/HER2− MBC,3 and the prognosis for these patients is poor.