ESR1 and breast cancer: Our results indicate that mQTL sites may be sensitive to environmental impacts and cancer driving mechanisms in a cell-type specific manner (Figs. 1b and 4), and could thereby act as a proxy for the cumulative exposure to steroid sex hormones and other risk factors that are essential for the development of both ER+ (evidenced by the primary preventive effects of tamoxifen and aromatase inhibitors38,39) and ER- breast cancers (evidenced by the numerous observational and experimental studies40).