CD38 (cluster of differentiation 38) is a glycoprotein found on the surface of many immune cells (white blood cells), including CD4 , CD8 , B and natural killer cells. CD38 also functions in cell adhesion, signal transduction and calcium signaling.In humans, the CD38 protein is encoded by the CD38 gene which located on chromosome 4. CD38 is a multifunctional ectoenzyme that catalyzes the synthesis and hydrolysis of cyclic ADP-ribose (cADPR) from NAD⁺ to ADP-ribose. These reaction products are essential for the regulation of intracellular Ca²⁺.The CD38 protein is a marker of cell activation. It has been connected to HIV infection, leukemias, myelomas, solid tumors, type II diabetes mellitus and bone metabolism, as well as some genetically determined conditions.