Tamoxifen can induce short-term proliferation of mammary epithelial cells. (A) Ki-67-immunostained sections of mammary glands from oil- and tamoxifen-treated mice at selected time points for 10–week- and 14-week-old treated mice. Scale bars = 50 μm. (B) Frequency of Ki-67+ cells in mammary glands of mice treated with oil or varying doses of tamoxifen, and analyzed at multiple time points. (C) Representative immunofluorescent sections of mammary glands from oil- and tamoxifen-treated glands at time points Days 3 and 7. Shown is the expression of keratin 5 (yellow), ER (red), and Ki-67 (green). Nuclei were visualized with DAPI. Scale bar = 25 μm. (D) Proportion of proliferating ER− luminal cells, ER+ luminal cells, and basal cells in tamoxifen-treated glands. (*P < 0.05; **P < 0.01; ***P < 0.0001).