Many of the risks of reproductive factors are related to the effects of estrogen as demonstrated by the reduction in breast cancer incidence after an early oophorectomy, by inhibition of the estrogen receptor (ER) by using selective estrogen receptor modulators (SERMs) such as a tamoxifen or raloxifene [9], or by blocking estrogen synthesis by using aromatase inhibitors (AIs) such as exemestane [10] and anastrozole [11],[12]. Here, ESR1 is linked to breast cancer.