ESR1 and breast cancer: Gallen surrogated for breast cancer subtypes has identified five primary entities based on the molecular classification through immunohistochemical expression of ER, PR, HER2, and proliferation index Ki-67: luminal-A, luminal-B, HER2-negative, luminal-B HER2-positive, HER2 enriched, and TNBC (triple negative: lack of expression of ER, PR, and no overexpression of HER2) [7, 8].