ESR1 and breast cancer: In clinical practice, according to a recent molecular classification, immunohistochemical expression of Estrogen (ER) and Progesterone (PgR) receptors and HER2 allows to categorize breast cancers in different prognostic subgroups (luminal A, luminal B, HER2 overexpressing, and triple negative breast cancers [TNBC]), with distinct therapeutic approaches [20].