Effects of hormonal treatments on apoptosis in BALB/c-Trp53+/- mammary glands. (a) Levels of TUNEL-positive mammary epithelium from nulliparous mice were decreased significantly with dosage of Trp53 (Trp53+/+ > Trp53+/- > Trp53-/-) in both nulliparous and hormone-stimulated groups. Data from control groups and hormone-treated groups of mice were pooled to provide sufficient numbers for all genotypes. The nulliparous group represents placebo-treated and age-matched virgins. The hormone-stimulated group represents mammary tissues from mice treated with E+P and parous mice as described in Figure 2a. Bars indicate statistical differences between means: *P < 0.05; **P < 0.01.(b) The incidence of mammary tumors was monitored in both nulliparous and multiparous BALB/c-Trp53+/- female mice. The latency of spontaneous mammary tumors was increased in parous compared with nulliparous mice (P < 0.001). TUNEL, terminal uridine deoxynucleotidyl transferase dUTP nick-end labeling.