ESR1 and breast neoplasm: Breast tumors can be classified by molecular subtypes, depending on the hormonal receptor expression [1], the main subtypes are Luminal A with Estrogen Receptor alpha (ERα)/Progesterone Receptor (PR) expression and no expression of human epidermal growth factor receptor type 2 (HER2); Luminal B with ERα/PR and HER2 expression; HER2 positive with HER2 overexpression and no expression of ERα/PR, and triple-negative (TNBC) with no hormonal receptors expression [2].