There is strong evidence that BRCA1 limits estrogen-mediated tumorigenesis: Brca1 knockout mice show an enhanced proliferative response to estrogen treatment and accelerated development of preneoplastic mammary lesions (21), and the reduction of serum estrogen levels by oophorectomy protects carriers of the BRCA1 mutation against breast cancer (22). Here, BRCA1 is linked to breast cancer.