Prostate-specific antigen (PSA; also known as human kallikrein-related peptidase 3), a 237-amino-acid glycoprotein (33 kDa) that belongs to the kallikrein serine protease family, is produced by the glandular epithelium of the human prostate and secreted in seminal fluid. PSA plays a principal role in the proteolysis of semenogelins I and II from seminal coagulum. Prostate-specific antigen (PSA) is a serum marker that is widely used for the diagnosis of prostatic diseases. Measurement of the free fraction of total prostate-specific antigen (fPSA%) has been proposed as a useful addition to total PSA for the detection of prostate cancer.