Immortal cell populations contain LRECs. Glands and fat pads from nulliparous mice either used intact or implanted with immortalized, premalignant cell clones were labeled with 5-BrdU (green arrow) and 3H-TdR (orange arrow). Double-labeled cells (labeled with both 5-BrdU and 3H-TdR, black arrow, inset) were identified by using light microscopy and were found in both intact glands (a, c, e) and implanted fat pads (b, d, f). LRECs were found in all immortal, premalignant cells (b, NSP2; d, NSP3; f, SP3). Representative images are shown. Scale bars equal 20 μm.