PAP is a tissue- specific target antigen rather than a tumor-specific target antigen. Studies with specific monoclonal antibodies and RNA probes indicate that the antigen is strictly prostate-specific. Immunohistochemical studies reveal that the antigen is expressed by normal prostate tissue, and > 90% of all prostatic adenocarcinomas, but is not expressed by other tissues (Goldstein 2002). PAP is secreted by the prostate tumor cells in vivo, and an elevated serum level is found in most subjects with advanced prostate cancer (Kuriyama 1982). The cDNA for PAP has been isolated.