Mammary epithelial proliferation shown by Ki67 immunoreactivity (brown dots) in ovariectomized monkeys treated with (a) placebo (Con), (b) estradiol (E2), (c) E2 and progesterone (P4), (d) tamoxifen (Tam) and (e) E2 and testosterone (T). (f) Quantification of the Ki67 proliferation index. Proliferation is increased with E2 or E2 and P4 (E/P), while this increase is attenuated by the addition of T to E2 (E/T). All differences are statistically significant when compared to the placebo group. Data from Zhou and colleagues .