ESR1 and breast cancer: Immunohistochemically, basal-like carcinomas are accurately classified by showing negativity for estrogen, progesterone and Human Epidermal growth factor receptors (Estrogen receptors [ER], Progesterone receptors [PgR], and Human epidermal growth factor receptor 2 [HER2], respectively), as well as for expressing basal cytokeratin 5/6; this panel identifies basal-like breast cancers with 100% specificity and 76% sensitivity [8].