ESR1 and breast cancer: In a series of articles, Perou et al. and Sorlie et al. have identified five intrinsic molecular BC subtypes based on distinct gene expression patterns: “Normal breast-like”, estrogen receptor α positive (ESR+) “Luminal A” and “Luminal B”, and ER negative (ESR-) “Receptor tyrosine-protein kinase erbB-2 amplification positive (HER2+)/ESR-“ and “Basal-like” subtypes [1, 2].