ESR1 and breast cancer: According to the PAM50 signature, breast cancer has been classified into five molecular subtypes: Luminal A [estrogen receptor (ER)+ and progesterone receptor (PR)+, HER2-, low levels of Ki-67], Luminal B [ER+, HER2-, and either Ki-67 high or PR low], Luminal B-like (ER+, HER2+, any Ki-67, and any PR, HER2-enriched [ER-, PR- and HER2+], triple negative (TNBC) or basal-like [ER−, PR-, and HER2-] [7, 8].