ESR1 and breast cancer: Breast cancer is a complex and heterogeneous disease that can be divided into four intrinsic subtypes: Luminal A (50–60% of breast tumors), Luminal B (15–20% of breast tumors), Her2+ (15–20% of breast tumors) and triple negative breast cancer (TNBC, 15–20% of breast tumors), which are mainly defined by the expression of estrogen receptor (ER), progesterone receptor (PR) and Her2, as well as in Ki-67, EGFR and basal cytokeratines status (Dai X. et al., 2015).