Hormone receptor-positive/human epidermal growth factor receptor-negative (HR+/HER2−; positive for the estrogen and/or progesterone receptors [≥1% positive by immunohistochemistry (IHC)] and negative for HER2 [IHC0, IHC1+, and IHC2+ and in situ hybridization negative]) BC is the most common subtype of BC, representing approximately 70% of cases [2,3,4]. The gene discussed is ERBB2; the disease is breast cancer.