ESR1 and breast cancer: ERα-expressing luminal A and B breast tumor subtypes account for 70% of total breast cancers [2], and are treated by hormone therapy aiming either at selectively blocking ERα activity (Selective Estrogen Receptor Modulator—SERM), dampening ERα expression (Selective Estrogen Receptor Degrader—SERD) or suppressing production of endogenous E2 (Aromatase Inhibitor—AI, LH-RH agonists) [3].