Prostatic intraepithelial neoplasia (PIN) and invasive neuroendocrine (NE) carcinoma in the prostate of a genetically engineered mouse (GEM) made with the non-androgen dependent CR2 promoter and the SV-40 early region. Other GEM mice made with androgen dependent promoters and the SV-40 early region or SV-40 Tag gene develop PIN that progresses to invasive and metastatic NE carcinoma. Invasive adenocarcinomas have also developed in such GEM models. Several knockout and transgenic mouse models established with non-SV40 based strategies also develop prostatic neoplasia. A classification scheme sponsored by the NCI Mouse Models of Human Cancer Consortium is presented herein for prostate pathology in GEM. Standardized pathology classification of prostate dis-eases in GEM should facilitate application to human prostate research. For details, see the article by Shappell et al. on p. 2270 of this issue.