ESR1 and breast neoplasm: While on average only 7% of normal mammary epithelial cells express ER, more than 70% of breast tumours are ER-positive (ER+) which suggests that ER+ cells are more prone to oncogenesis, presumably due to their ability to respond to a variety of biological (endogenous estrogens) and environmental stimuli (such as steroid-like molecules) (2,3).