Kallikrein 3, commonly known as prostate specific antigen (PSA), is a serine protease of the human tissue Kallikrein gene family. The human tissue kallikrein (KLK) gene family contains 15 members that play important roles in cancer. Kallikrein-3, called prostate specific antigen (PSA), is a glycoprotein enzyme encoded in humans by the KLK3 gene. PSA is secreted by the epithelial cells of the prostate gland, hydrolyzes semenogelin-1 thus leading to the liquefaction of the seminal coagulum. PSA is an established tumor marker that aids in the diagnosis, staging, and follow up of prostate cancer.