BRCA1 and breast cancer: Since estrogen is an intrinsic source of the induction of DNA damage in mammary epithelial and tumor cells under physiological condition, we then transplanted p18−/− (Gata3+/+;Brca1+/+), p18−/−;Gata3+/− (Gata3-deficient), and p18−/−;Brca1+/− (Brca1-deficient) mammary tumor cells into MFPs of the recipient mice who were simultaneously administered estrogen or vehicle.