The prostate-specific antigen (PSA), also known as Kallikrein-related peptidase 3 (KLK3), is a single-chain glycoprotein and protease synthesized by the prostate epithelial cells and remains the most evaluated serum biomarker used for early prostate cancer screening, clinical staging, and therapy response monitoring. The gene discussed is KLK3; the disease is Familial prostate cancer.