The enzyme Dipeptidyl peptidase-4 (DPP-4) also part of the CD26 surface region is associated with immune regulation, signal transduction and apoptosis. It is a rather indiscriminate enzyme for which at least 62 substrates are known. Furthermore it appears to work as a suppressor in the development of cancer and tumours. DPP-4 plays a major role in glucose metabolism. It is responsible for the degradation of incretins such as GLP-1. DPP-4 also binds the enzyme adenosine deaminase specifically and with high affinity. The significance of this interaction has yet to be established.