LRECs present in the mammary fat pads of pregnant and nulliparous mice implanted with immortalized, premalignant cell populations express ER-α and PR and incorporate 3H-TdR into their nuclei. Immunohistochemistry for ER-α ((a, c), red arrow) and PR ((b, d), red arrow), and labeling for 3H-TdR, was performed on mouse mammary glands (inguinal) of nulliparous mice (a, b) or pregnant (c, d) implanted with immortalized, premalignant cells. 3H-TdR was incorporated into the nucleus of some cells expressing the steroid receptors (green arrow, inset). Representative images are shown. Scale bars equal 20 μm.