ESR1 and breast cancer: Characterized by high heterogeneity, BC is classified into luminal A, normal-like, luminal B, HER2-enriched, and triple-negative breast cancer (TNBC) subtypes based on hormone receptor expression (estrogen receptor [ER], progesterone receptor [PR]), the proliferation marker Ki-67, and human epidermal growth factor receptor 2 (HER2) status.