Cell proliferation is decreased and apoptosis is increased in mammary epithelium from MK-PTEN and WT mice during pregnancy. At the indicated stages, pregnant MK-PTEN and WT mice were injected with BrdU and sacrificed 2 hours later, as described in Methods. (a) BrdU-labeled nuclei were detected by immunostaining, and apoptotic nuclei were stained by TUNEL. (b) About 3,000 nuclei were counted, and the percentage of labeled epithelial cells was determined at 9 (P-9), 14 (P-14), and 20 (P-20) days of pregnancy. Data are expressed as the percentage of labeled cells ± SD. Averages were compared by t test. Averages were compared by t test. *P < 0.05, ***P < 0.001. WT, black bars; MK-PTEN, white bars.