Prostate-specific antigen (PSA), also known as human kallikrein 3 (hK3), according to the approved new nomenclature of the human kallikrein family (Diamandis et al, 2000a), is used for early detection and monitoring of prostate cancer (Bilhartz et al, 1991; Oesterling, 1991; Diamandis et al, 2000b). The gene discussed is KLK4; the disease is prostate cancer.