ESR1 and breast cancer: In breast cancer, luminal type is characterized by high expression of estrogen receptor (ER) and/or progesterone receptor (PR), basal type by low expression of ER, PR and human epidermal growth factor receptor 2 (HER2), and high expression of basal epithelial genes [2], and triple negative (TNBC) type by low expression of all three, ER, PR and HER2 [3].