ESR1 and breast cancer: In routine clinical practice, BC can be classified into prognostic and predictive subtypes based on the immunohistochemical (IHC) expression of the estrogen receptor (ER), the progesterone receptor (PgR) and the human epidermal growth factor receptor type 2 (HER2), with or without in situ hybridization of the latter, in equivocal 2 + tests2.