ESR1 and breast cancer: Breast cancers are categorized into five different subtypes,luminalA/B, HER2-positive (HER+), basal-like, claudin-low, andnormal breast-like, based on the expression levels of estrogen andprogesterone receptors (ER and PR), HER2, cytokeratins 5/6, and claudins3/4/7.54−56