ESR1 and breast cancer: Breast cancer is a heterogeneous disease classified into four molecular subtypes based on the presence of hormone receptors: luminal A (progesterone and estrogen receptor positive (PR+/ER+) and human epidermal receptor 2 negative (HER2-)), luminal B (PR+/ER+/HER2+), HER2 (PR-/ER-/HER2+), and triple-negative (PR-/ER-/HER2-).