Illustration of prostate-specific antigen (PSA), also known as gamma-seminoprotein or kallikrein-3 (KLK3). PSA is a protein secreted by the prostate, a gland that forms part of the male reproductive system. PSA liquefies semen, increasing sperm mobility. It may also have a role in breaking down cervical mucus, aiding entry of the sperm during fertilisation. Abnormally high levels of PSA in the blood can be an indicator of prostate cancer or other issues, but this is not a reliable indicator on its own.