ESR1 and breast cancer: For instance, certain SNPs located in ERα, ERβ, HSD17B1, COMT and CYP1B1 genes have been associated with breast cancer risk among premenopausal or postmenopausal women [9-12] or in some strata of other estrogen-related factors such as parity, hormone replacement therapy (HRT) use, age at menarche or body mass index (BMI) [9,13-15].