We have isolated a prostate-specific gene (NKX3.1) in humans that is homologous to theDrosophilaNK homeobox gene family. Northern blot analyses indicate that this gene is expressed at high levels in adult prostate and at a much lower level in testis, but is expressed little or not at all in several other tissues. In an androgen-dependent prostate carcinoma line, LNCaP,NKX3.1mRNA is expressed at a basal level that was increased markedly upon androgen stimulation; theNKX3.1mRNA was undetectable in several other human tumor cell lines including two androgen-independent prostate carcinoma lines. TheNKX3.1gene maps to chromosome band 8p21, a region frequently reported to undergo a loss of heterozygosity associated with tissue dedifferentiation and loss of androgen responsiveness during the progression of prostate cancer. Based on these data we propose thatNKX3.1is a candidate gene for playing a role in the opposing processes of androgen-driven differentiation of prostatic tissue and loss of that differentiation during the progression of prostate cancer.