CD26 is a 110 kD type II membrane protein also known as ADA-binding protein and dipeptidyl peptidase IV (DPPIV). It is a member of the peptidase and ectoenzyme family. CD26 is expressed on the membrane of mature thymocytes, T lymphocytes (upregulated upon activation), B cells, NK cells, and macrophages. CD26 cleaves off N-terminal X-Pro and X-Ala dipeptides from polypeptides. It plays an integral role as a costimulatory molecule in T cell activation. CD26 may interact with extracellular matrix proteins such as fibronectin or collagen, CD45 and ADA.