NKX3.1 is a prostate-specific gene encoding a transcription factor essential in normal prostate development and carcinogenesis. It is a prostatic tumor suppressor gene located on chromosome 8p21.2, which frequently undergoes a loss of heterozygosity. NKX3.1 expression is highly restricted in prostate epithelial cells and, therefore, can be used as a diagnostic biomarker for prostate cancer and other metastatic lesions of prostatic origin. Furthermore, NKX3.1 shows better sensitivity than Prostate-Specific Antigen (PSA) for identifying metastatic prostatic adenocarcinoma. This suggests that immunohistochemical staining of NKX3.1, along with other prostate-restricted markers, may be valuable for the definitive determination of prostatic origin in poorly differentiated metastatic carcinomas.