ESR1 and breast cancer: In spite of big differences in morphology, growth, survival, migration, invasiveness, and metastasis, breast cancer cells are commonly divided into 4 subtypes, luminal A (ER+ and/or PR+, HER2−), luminal B (ER+ and/or PR+, HER2+), HER2 (ER−, PR−, HER2+), and triple-negative/basal-like (ER−, PR−, HER2−), based on expression of three cell surface receptors, estrogen receptor (ER), progesterone receptor (PR), and HER2/neu receptor (HER2).