NKX3.1 is a prostate specific androgen-regulated homeobox gene located on chromosome 8p. It is difficult to distinguish between high grade prostate adenocarcinoma and high grade infiltrating urothelial carcinoma using hematoxylin and eosin stained specimens. Current prostate adenocarcinoma markers such as prostate specific antigen (PSA) and prostate specific acid phosphatase (PSAP) are very useful in determining prostate origin of prostate cancer in other sites but have lower sensitivity when identifying poorly differentiated compared to well differentiated cases. NKX3.1 is a sensitive and specific tissue marker of prostate adenocarcinoma and can be used to help distinguish it from urothelial carcinomas.  Currently, thrombomodulin and uroplakin are used to identify tumors of urothelial origin; however, their sensitivities are suboptimal. NKX3.1 is a sensitive and specific tissue marker of prostate adenocarcinoma and can be used to help distinguish it from urothelial carcinomas as well as tumors of unknown primary.