HR and breast cancer: Clinically, breast cancer is categorized into the three following major subtypes based on the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2, also known as ERBB2): hormone receptor-positive breast cancer (HR+ BC), HER2-positive breast cancer (HER2+ BC), and triple-negative breast cancer (TNBC) [2].