Tumor-free survival in BALB/c mice bearing transplants of Trp53+/- mammary epithelium. Tumor incidence was also determined for outgrowths of Trp53+/- mammary tissue transplanted into wild-type hosts. The timeline of treatments is summarized graphically in the upper panel. Donor mammary tissues from Trp53+/- mice (described in Figure 8a) were transplanted into epithelium-free mammary fat pads of 3-week-old wild-type mice. Beginning at 10 weeks of age, host mice received a 1.2-Gy dose of ionizing radiation once a week for 4 weeks to induce mammary tumors as described previously . Mice were palpated weekly for mammary tumors until 65 weeks old. The incidence and latency of mammary tumors did not differ between epithelial outgrowths from the Trp53+/- mammary tissues from E+P-treated donors and nulliparous donors (P > 0.05). E+P, estradiol and progesterone; IR, ionizing radiation; Trp53, transformation-related protein 53 (gene in mouse encoding the p53 tumor suppressor protein); TUNEL, terminal uridine deoxynucleotidyl transferase dUTP nick-end labeling.