In 2000, the first-generation cDNA microarrays defined basal-like breast cancer as a subtype of breast cancer, which shows low expression level of estrogen receptor (ER) and associated genes, no expression of human epidermal growth factor receptor 2 (HER2) and strong expression of breast basal cell keratins 5/6 and 17 [3]. Here, ESR1 is linked to breast cancer.