Studies in genetically modified mice revealed that: 1) a PR knockout mouse shows dramatically reduced susceptibility to carcinogenesis [15], 2) progesterone increases genomic instability in p53 null mouse models of breast cancer [16], and 3) treatment of Brca-1-deficient mice with the anti-progestin mifepristone (RU486) prevented mammary tumorigenesis [17]. Here, PGR is linked to breast cancer.