Increases in radiation-induced apoptosis are retained in E+P-treated mice after ovariectomy. (a) Mice were treated with E+P or placebo at 6 to 8 weeks of age. At 10 weeks of age, mice were ovariectomized and endogenous hormones were allowed to clear for 2 weeks. All mice received a 5-Gy dose of radiation 6 hours prior to tissue harvest. (b) Radiation-induced apoptosis was increased in mammary tissues from E+P-treated mice compared with the placebo-treated controls (*P < 0.01). (c) TUNEL-positive epithelial cells were minimal in ducts from nulliparous ovariectomized mice (i) but were detected frequently in tissues from mice that were E+P-treated and ovariectomized (ii). All images were taken at × 400. E+P, estradiol and progesterone; IR, ionizing radiation; OVX, ovariectomized; TUNEL, terminal uridine deoxynucleotidyl transferase dUTP nick-end labeling.