The homeodomain-containing transcription factor NKX3. 1 is a putative prostate tumor suppressor that is expressed in a largely prostate-specific and androgen-regulated manner. Loss of NKX3. 1 protein expression is common in human prostate carcinomas and prostatic intraepithelial neoplasia (PIN) lesions and correlates with tumor progression. Disruption of the murine Nkx3. 1 gene results in defects in prostate branching morphogenesis, secretions, and growth. To more closely mimic the pattern of NKX3. 1 loss that occurs in human prostate tumors, we have used Cre-and loxP-mediated recombination to delete the Nkx3. 1 gene in the prostates of adult transgenic mice. Conditional deletion of one or both alleles of Nkx3. 1 leads to the development of preinvasive lesions that resemble PIN. The pattern of expression of several biomarkers (Ki-67, E-cadherin, and high-molecular-weight cytokeratins) in these PIN lesions resembled that observed in human cases of PIN. Furthermore, PIN foci in mice with conditional deletion of a single Nkx3. 1 allele lose expression of the wild-type allele. Our results support the role of NKX3. 1 as a prostate tumor suppressor and indicate a role for this gene in tumor initiation.