HR and breast cancer: However, the optimal treatment of breast cancers has become more precise according to molecular subtypes, which are classified based on hormone receptor (HR) and human epidermal growth factor receptor 2 (HER2) into four primary molecular subtypes, which include HR+/HER2− (Luminal A), HR+/HER2+ (Luminal B), HR−/HER2− (i.e., triple-negative breast cancer, TNBC), and HER2+.