ESR1 and breast cancer: Additionally, multiple studies confirmed the usefulness of joint immunohistochemical (IHC) evaluation of ERα, PgR, human epidermal growth factor receptor 2 (HER2), and Ki67, which enables subclassification of BC into surrogate intrinsic phenotypes, with the cut-off value discriminating between luminal A-like and luminal B-like tumors proposed at 20% of cells positive for PgR expression [6].