Prostate-specific antigen (PSA) is a serine protease member of the human glandular kallikrein family. It is synthesized in the prostate ductal and acinar epithelium and diffused into serum. It is found in normal, hyperplastic, and malignant prostate tissue. Low expression of PSA has been reported in other normal or tumor tissues such as urethral, periurethral, and perianal glands, salivary duct carcinoma, and rare mammary carcinomas. Although low PSA expression has been found in other tissues, PSA is still a specific and sensitive marker for immunohistochemical analysis of tumors with prostate epithelial cell differentiation. It is valuable in the identification of metastatic tumors of prostatic origin.