ESR1 and breast carcinoma: Biomarker assessment of estrogen receptor (ER)-, progesterone receptor (PR)-, Ki-67, and HER2 status with immunohistochemistry (IHC) is used to classify breast cancer by intrinsic/surrogate subtypes according to international WHO guidelines: Luminal A-like, Luminal B-like (HER2- or HER2+), HER2+ (non-luminal) and Triple-negative [6].