ESR1 and breast cancer: The first gene expression-based classification of breast cancer into intrinsic subtypes was published in 2000 [5] and identified estrogen receptor (ER) positive (ER+) subtypes Luminal A and B, and ER negative (ER-) subtypes basal-like and human epidermal growth factor receptor 2-enriched (HER2-enriched) [1, 6].