Dipeptidyl peptidase IV (DPPIV/CD26) is a serine exopeptidase that releases Xaa-Pro dipeptides from the N-terminus of proteins and peptides. It plays an important role in T-cell costimulation, chemokine biology, type II diabetes and tumor biology. There are at least 15 DPPIV-related human genes. Some function as enzymes, including DPP8, DPP9, fibroblast activation protein (FAP), prolyl endopeptidase (PREP), and N-acylaminoacyl-peptide hydrolase (APEH) while others (DPP6/DPPX and DPP10/DPRP3) may have lost their protease properties but acquired novel functions. For example, DPP6 is a critical component of neuronal A-type K+ channels.