Prolyl_oligopeptidase_S9 are serine protease of the clan S family S9 in MEROPS database. Previously this family contained also all the subfamilies ACPH_Peptidase_S9, Glutamyl_Peptidase_S9, DPP4N_Peptidase_S9, S9N_Peptidase_S9. Now it contains entries that do not fit in these well defined sub families. Few proteins characterized in this family. Aspergillus fumigatus DPP5 and DPP4 (Beauvais et al.) DPP5 unique substrate specificity limited to the hydrolysis of X-Ala, His-Ser, and Ser-Tyr dipeptide and not post Proline