ESR1 and breast cancer: A major challenge in the treatment of breast cancer is its high heterogeneity from patient to patient which initiated its classification into three major molecular subtypes, according to estrogen receptors (ER), progesterone receptors (PR) and HER2: hormone receptor positive with luminal A (ER+PR+HER2−) and luminal B (ER+PR+HER2+) phenotypes, HER2 positive (ER−PR−HER2+) and triple negative/basal-like (ER−PR−HER2−) [17,18,19].