HR and breast cancer: Clinically, breast cancer is primarily categorized into three main subtypes based on receptor expression: hormone receptor (HR)-positive and HER2-negative, which makes up about 70% of cases; HER2-positive, with or without HR expression, accounting for 15–20% of breast cancers; and triple-negative breast cancer, defined by the absence of both HR and HER2 expression, representing 10–15% of breast cancers [2].