ESR1 and breast cancer: At the molecular level, based on hormone receptor status, breast cancers are categorized as Luminal A (estrogen receptor (ER)+, progesterone receptor (PR)+, and human epidermal receptor 2 (HER2)-); Luminal B (ER+ and/or PR+, HER2+/−); HER2 enriched (ER-, PR- and HER2+); and Basal-like including triple negative (ER-, PR- and HER2-) breast cancers [3].