Previously called Prolyl_oligopeptidase_S9 (but are not only prolyl peptidase) Peptidase_S9 are serine protease of the clan S family S9 in MEROPS database. It contains: - prolyl oligopeptidase (EC 3.4.21.26) (also called post-proline cleaving enzyme);- Escherichia coli protease II (EC 3.4.21.83) (oligopeptidase B) (gene prtB);- dipeptidyl petidase IV related proteins(EC 3.4.14.5) (DPP IV) important for the activation or inactivation of extracellular endocrine paracrine and autocrine peptides. DPPIV is synonymous with CD26 (important in HIV infection). Some members lack active serine and are probably not enzymes. The prototype structure has a beta propeller which guide substrates to the active site;- Yeast vacuolar dipeptidyl aminopeptidase A;- Acylamino-acid-releasing enzyme (EC 3.4.19.1) (acyl-peptide hydrolase).