CD38 is a 42 kD glycoprotein, also known as T10. It is an ADP-ribosyl hydrolase, expressed on B cells, NK cells, a subset of T cells, brain, muscle, and kidney. In mouse, CD38 expression is downregulated on germinal center B cells and plasma cells, whereas this is not the case for humans. By functioning as both a cyclase and a hydrolase, CD38 mediates lymphocyte activation, as well as adhesion and metabolism of cADPR and NAADP. CD31 is the ligand of CD38.