A hematoxylin-eosin stained section from the anterior prostate of an Nkx3.1 homozygous mutant (Nkx3.1-/-) mouse at 19 months of age. This section is typical of aged Nkx3.1-/- mutant prostates, which display multilayered hyperplastic and severely dysplastic epithelium, with little luminal space or secretory material. In addition to these histological features, which are reminiscent of human prostatic intraepithelial neoplasia (PIN), Nkx3.1 mutant prostates display other additional histopathological features characteristic of early-stage human prostate cancer, including alterations of the basal epithelial layer and an increased ratio of epithelial-stromal cells. These and other observations demonstrate that Nkx3.1 mutant mice model key features of human prostate cancer initiation. For details, see the article by Kim et al. on page 2999 of this issue.