CPXM (carboxypeptidase X, member 1) belongs to the peptidase M14 family. However, no carboxypeptidase activity has yet been detected. It may be involved in cell-cell interactions.Members of the M14 metallocarboxypeptidase protein family serve many diverse functions and are divided into three subfamilies based on structure, function and amino acid sequence similarity. Belonging to the N/E subfamily, CPXM (metallocarboxypeptidase CPX-1) is a 734 amino acid protein that contains a F5/8 type C domain and likely binds one zinc ion per subunit. Most members of the N/E subfamily contain several domains, including an active carboxypeptidase domain and signal peptide, and are thought to function mostly in protein-protein interactions and/or protein-membrane interactions, thereby targeting the protein to specific locations within the secretory pathway. CPXM is a unique member of this subfamily in that it does not appear to exhibit any enzymatic activity due to lack of several active-site residues that are present in the catalytic domain of other members of the N/E subfamily. Studies showing that CPXM expression is regulated during osteoclastogenesis suggest that CPXM may play a role in osteoclast differentiation. There are two isoforms of CPXM which are a result of alternative splicing events.