The following examples have been widely used for this purpose: the carcinoembryonic antigen (CEA) for adenocarcinoma in general, the squamous cell carcinoma antigen for various squamous cell carcinomas, a protein induced by the absence of vitamin K antagonist-II for hepatocellular carcinoma (HCC), the alpha-fetoprotein for yolk sac tumors and HCC, the cancer antigen 125 for ovarian cancer and the prostate-specific antigen (PSA) for prostate cancer (PCa). The gene discussed is KLK3; the disease is prostate cancer.