ESR1 and breast cancer: Based on the histology and molecular analysis, five main intrinsic or molecular subtypes have been identified: luminal A BC (estrogen-receptor [ER] and/or progesterone-receptor [PR] positive, human epidermal growth factor receptor [HER2] negative, and Ki-67 ≤ 14%), luminal B BC (ER and/or PR positive, and either HER2 positive or HER2 negative with PR < 20% and Ki-67 > 14%), triple-negative/basal-like BC (TNBC) (ER, PR and HER2 negative), HER-2-positive BC (ER and PR negative, and HER2 positive), and normal-like BC (similar to normal mammary gland) [2–5].