Fig. 4. Impaired cell proliferation in ductal and alveolar epithelium of MTA1-TG mice. (A) BrdU incorporation into the nuclei of mammary epithelial cells from wild type and MTA1-TG virgin at 12 weeks of age and on day 10 of pregnancy. (B) Quantitation of BrdU incorporation in the nuclei of mammary epithelial cells at 6 and 12 weeks in virgin mice, and at day 10 and day 15 of pregnancy in wild-type and MTA1-TG mice. Five thousand cells per mouse were counted, and six mice per line were examined. The proliferative index was calculated as follows: (number of BrdU-labeled cells/total number of cells)x100. Student's t-test, P<0.01. Error bars indicate s.e.m.