Considering that tamoxifen is an estrogen-receptor (ER) antagonist and is a commonly used drug in the treatment of breast cancer to inhibit the proliferation of estrogen receptor+ (ER+) breast cancer cells [10], we investigated the effects of a range of doses of tamoxifen on the distribution and cell-division kinetics of the different mammary cell populations in the intact adult mouse mammary gland. Here, ESR1 is linked to breast cancer.