The prostatic epithelium and the periurethral glands’epithelial lining are the main producers of the 33 kDaprotein known as prostate-specific antigen (PSA). 1 Bothhealthy and malignant prostatic tissue have high levels ofPSA expression. When employing IHC techniques toidentify the prostatic origin of carcinomas in non-prostatetissues (metastatic disease), anti-PSA is most helpful. Theprostate epithelium produces PSA, a chymotrypsin-likeserine protease (kallikrein family).