The Dipeptidyl peptidase 9 gene encodes a protein that is a member of the S9B family in clan SC of the serine proteases. It has been shown to have post-proline dipeptidyl aminopeptidase activity. Dipeptidyl peptidases are involved in the regulation of the activity of their substrates and have been linked to a variety of diseases including type 2 diabetes, obesity and cancer. Other diseases associated with DPP9 include pulmonary fibrosis and nasopharyngitis. Here, DPP9 is linked to nasopharyngitis.