ESR1 and breast carcinoma: In order to provide a tailored treatment, breast carcinomas can be subdivided into five different molecular subtypes based on the presence of hormone receptors (HR, corresponding to estrogen receptor, ER, and progesterone receptor, PR), human epidermal growth factor receptor 2 (HER2), and the protein Ki-67, which serves as a marker for cellular proliferation: luminal A, normal-like (similar to luminal A, but with a slightly worse prognosis), luminal B, HER2-enriched, and triple-negative, which is also referred to as basal-like [2].