Proliferation and apoptosis in morphologically normal alveoli (a,b), ducts (c,d), and epithelial hyperplasias (e,f) in mammary glands of virgin females at 6 months of age, as well as end stage for transgenic animals (nontransgenic (nonTG), NRL-PRL 1647-13, NRL-PRL 1655-8). Epithelial hyperplasias were infrequent in glands of nontransgenic and line 1647-13 mice at 6 months of age, so only end-stage data for the transgenic lines are shown. BrdU labeling and apoptotic indices were determined from multiple structures in five animals of each genotype as described in Materials and methods, and expressed as means±s.d. Different lower case letters denote statistical differences among groups as determined by the Kruskal–Wallis test for nonparametric data, followed by the Mann– Whitney post test (P<0.05)