ESR1 and neoplasm: This molecular classification includes a variety of subtypes with correlation to the histological subtypes described above, being the most common luminal A (ER- and/or PR-positive and HER2-negative), luminal B (ER- and/or PR-positive and HER-positive, as well as high Ki67 expression), HER2 overexpression (ER- and PR-negative and HER2-positive) and a group of tumours that lack ER, PR and HER2 expression—amongst which, the basal-like subtype (expression of basal markers, such as keratins 5, 6, 14 and 17 and epidermal growth factor receptor (EGFR/HER1)) represents 60–90% of TNBC cases [3].