Prostate Specific Antigen (PSA, KLK3) is a serine glycoprotein protease found in prostate tissue that is used in the diagnosis of prostate tumors, where levels of the protein are often elevated. Prostate Specific antigen is freely present in serum but may also be complexed with inhibitors such as alpha 1-antichymotrypsin (known as the PSA-ACT complex). When compared to free PSA, the PSA-ACT complex has higher levels in cancer than in benign tumors, and PSA-Act Complex antibodies may be more sensitive for cancer diagnosis than those that strictly target PSA. References: Cancer Res. 1991 Jan 1;51(1):222-6, PMID: 1703033; Prostate. 2013 Jan;73(2):219-26, PMID: 22806587; BJU Int. 2005 Apr;95(6):761-5, PMID: 15794778