Prostate-specific antigen (PSA), also known as gamma-seminoprotein or kallikrein-3 (KLK3), is a member of the kallikrein-related peptidase family and is secreted by the epithelial cells of the prostate gland. PSA is present in small quantities in the serum of men with healthy prostates, but is often elevated in the presence of prostate cancer or other prostate disorders. PSA is now used to indicate the presence of semen in forensic serology. The semen of adult males has PSA levels far in excess of those found in other tissues; therefore, a high level of PSA found in a sample is an indicator that semen may be present. Because PSA is a biomarker that is expressed independently of spermatozoa, it remains useful in identifying semen from vasectomized and azoospermic males.