The Million Women Study [6] and previous studies have shown that in premenopausal women breast cancer risk decreases with increasing BMI, while it increases with BMI in postmenopausal women who do not practice hormone replacement therapy, possibly owing to heightened levels female sex hormones in circulation, caused by increased estradiol creation from androgenic precursors in adipose tissue through an increase in activity of the aromatase enzyme. This evidence concerns the gene CYP19A1 and breast cancer.