X-Prolyl Aminopeptidase (XPNPEP1) is a proline-specific metalloaminopeptidase that specifically catalyzes the removal of any unsubstituted N-terminal amino acid that is adjacent to a penultimate proline residue. Because of its specificity toward proline, it has been suggested that X-Prolyl Aminopeptidase is important in the maturation and degradation of peptide hormones, neuropeptides, and tachykinins, as well as in the digestion of otherwise resistant dietary protein fragments, thereby complementing the pancreatic peptidases. X-Prolyl Aminopeptidase is a member of the M24 family of metalloproteases, which also contains methionine aminopeptidases, X-Pro dipeptidase, aminopeptidase P2, aminopeptidase P homolog, proliferation-associated protein 1, and suppressor of Ty homolog or chromatin-specific transcription elongation factor large subunit. It is a soluble enzyme, in contrast to the GPI-anchored Aminopeptidase P2 encoded by XPNPEP2. Deficiency of X-Prolyl Aminopeptidase results in excretion of large amounts of imino-oligopeptides in urine. Human Aminopeptidase P1 is widely expressed. The amino acid sequence of human X-Prolyl Aminopeptidase is 99%, 97%, 95%, 74% and 73% identical to that of canine, bovine, mouse/rat, Xenopus and zebrafish, respectively.