PSA, or Kallikrein related peptidase 3, is a human gene encoded by KLK3. It may also be known as: Prostate-specific antigen; Gamma-seminoprotein; Seminin; Kallikrein-3; P-30 antigen; Semenogelase; KLK2A1; kallikrein-related peptidase 3. The encoded protein is an enzyme with an amino acid length of 261 and a mass of 28.7 kDa. Other isoforms may exist. PSA is a member of the Peptidase S1 family, Kallikrein subfamily.