Currently, breast cancer has the highest incidence and the second highest cancer mortality among US women.9 Hormone-receptor–positive, ERBB2 (formerly HER2/neu)-negative metastatic breast cancer (HR-positive, ERBB2-negative MBC) is the most common subtype of metastatic breast cancer and is considered to be incurable.10 HR-positive, ERBB2-negative MBC has been studied through carefully designed RCTs of cytotoxic chemotherapy, endocrine therapy, targeted therapy, and their combinations since the 1970s. Here, ERBB2 is linked to breast cancer.