Effects of hormonal treatments on hyperplasia prevalence in mammary glands of PR-A transgenic mice. Five to six tissue sections, corresponding to each group and representing mammary glands from two to three PR-A transgenic mice, were stained with H&E and a minimum of 100 ducts per section were counted. Each mammary gland from each mouse was analyzed in duplicate. Data is presented as percentage of total ducts and represent the mean ± standard error of the mean. Ovariectomy led to a decrease in the percentage of hyperplasias (intact vs. ovariectomized (Ovx), *P ≤ 0.01), which was partially recovered by administration of estradiol (Ovx vs. Ovx+E, *P ≤ 0.01) and was completely recovered by treatment with estradiol 1 μg and progesterone 1 mg (Ovx vs. OvxE+P, *P ≤ 0.001). On the other hand, treatment with ICI 182,780 also led to a decrease in the percentage of hyperplasias (intact vs. intact+ICI 182,780, *P ≤ 0.001), but neither the antiprogestins RU486 or ZK 98,299 had an effect. Ovx+P, ovariectomized mouse treated with progesterone 1 mg.