CD26 is a 110 kD type II membrane protein also known as ADA-binding protein and dipeptidyl peptidase IV (DVPPIV). It is a member of the peptidase and ectoenzyme family. CD26 is expressed on the membrane thymocytes, T lymphocytes (unpregulated upon activation), B cells, NK cells, macrophages, epithelial cells, and endothelial cells. Rat CD26 (unlike in humans) does not bind ADA, but may interact with extracellular matrix proteins such as fibronectin or collagen. CD26 plays an integral role as a costimulatory molecule in T cell activation.