Prostate-specific antigen (PSA) is a single-chain glycoprotein of 237 amino acids containing approximately 8% carbohydrate. It is a serine protease produced almost exclusively by prostatic epithelial cells. Immunohistochemically PSA is expressed in the highly specialized apically-superficial layer of female and male secretory cells of the prostate gland and is readily demonstrated in adenocarcinomas of the prostate in about 99% of the cases.