ESR1 and breast cancer: Molecular characterization has shown that breast cancer is a highly heterogeneous disease that can be divided into at least four well-defined subtypes: the hormone receptor positive subtypes, luminal A and luminal B, the HER2 subtype enriched for cases with HER2 amplification, and the basal-like subtype which usually lacks expression of the estrogen (ER), progesterone (PR), and HER2 receptors (so called “triple-negative”) [8–10].