ESR1 and breast cancer: As a result of heterogeneity, cancer is classified into various molecular subtypes based primarily on their gene expression patterns of hormone receptors and growth factors.12,13 For the molecular classification of breast cancer into luminal A, luminal B, HER2-enriched, and triple-negative subtypes, three receptors – estrogen receptor (ER), progesterone receptor (PR), and human growth factor receptor 2 (HER2) are used.13