Cell proliferation as determined by immunohistochemical staining of Ki-67 in the ventral and dorsolateral prostates of SV-40 Tag and non-transgenic rats fed AIN-76A diet. Each age group had at least 3 transgenic and 2 non-transgenic animals per group. Values are presented as total number of proliferating epithelial cells divided by total number of epithelial cells × 100 ± SEM. Picture inserts are examples of brown staining for Ki-67 in ventral prostates of 32 week old rats. A) Example of rare proliferating cells (brown staining – arrow) for KI-67 in ventral prostate from 12 week old non-transgenic rat. B) Numerous proliferating Ki-67 positive cells (brown stain) in ventral prostate of a SV-40 Tag rat; a prostatic duct with high grade intra-epithelial lesion showing marked tufting, papillary formation, nuclear crowding and high nuclear cytoplasmic ratio.