This approach was initially based upon the discovery by Hans Lilja at the University of Lund, Sweden, that Prostate-Specific Antigen (PSA), besides being a serum marker for prostate cancer detection, is a unique prostate differentiation restricted chymotrypsin-like serine protease whose enzymatic activity, once secreted by normal or malignant prostate epithelial cells into the serum, is inhibited by serum inhibitors [47,48,49]. The gene discussed is KLK3; the disease is prostate cancer.