Aminopeptidase P (APP) is a hydrolase specific for N-terminal imido bonds, which are common to several collagen degradation products, neuropeptides, vasoactive peptides, and cytokines. A membrane-bound and soluble form of this enzyme (XPNPEP2) have been identified as products of two separate genes. XPNPEP2, the X-linked gene that encodes membranous aminopeptidase P (APP), has been reported to associate with APP activity. The membrane aminopeptidase P (XPNPEP2) is largely limited in distribution to endothelia and brush border epithelia. APP and XPNPEP2 contain homologous blocks of sequence common to members of the "pita bread-fold" protein family, of which Escherichia coli methionine aminopeptidase is the prototype. The C-2399A variant in XPNPEP2 is associated with reduced APP activity and a higher incidence of AE-ACEi. XPNPEP2 mRNA was detected in fibroblasts that carry the translocation, suggesting that this gene at least partially escapes X inactivation. XPNPEP2 is a candidate gene for premature ovarian failure (POF).