ESR1 and breast carcinoma: Breast cancer is a highly heterogeneous tumor that can be further categorized into five subtypes depending on their intrinsic gene expression profiles: Luminal A, Luminal B, human epidermal growth factor receptor 2-enriched (HER2+ve), basal-like and normal-like [14], of which Luminal A, Luminal B and normal-like are ERα-positive (ERα+ve), HER2+ve are ERα negative (ERα-ve) and basal-like are often ERα, progesterone receptor (PR) and HER2 negative (i.e., triple negative breast cancer; hereafter referred as TNBC).