Prostate specific antigen (PSA), or Kallikrein-Related Peptidase 3 (KLK3), is a secreted protease produced in prostate glandular epithelium that is thought to function in the liquefaction of seminal fluid. PSA is a highly sensitive and specific diagnostic marker for recurrent prostate carcinoma after prostatectomy. It is also useful for confirming suspected prostatic origin in the setting of metastatic carcinoma. PSA is expected to have cytoplasmic staining in normal prostate tissue, periurethral and perianal glands, the bladder, the urethra and in seminal vesicles. PSA can also be found in tumors of the colon, liver, lung, parotid, adrenal, and ovary, and in metaplasias of the bladder and nodular hyperplasia or prostatitis. References: Oesterling, 1991; 1988; Stamey, 1987.