The AT-1 antibody reacts with CD38, an ~45 kDa type II integral membrane glycoprotein that is part of the ADP-ribosyl cyclase family. CD38 is widely expressed at variable levels on hematopoietic cells including early B and T cell lineages, activated B and T cells, NK cells, monocytes, and dendritic cells. It is strongly expressed in plasma cells. CD38 is also expressed in some non-hematopoietic tissues such as brain, kidney, muscle, and pancreas. CD38 is an ectoenzyme that acts as both an ADP-ribosyl cyclase and ADP-ribose (ADPR) hydrolase for the synthesis and hydrolysis of cyclic ADPR (cADPR) and NAADP. cADPR acts as a second messenger for intracellular calcium mobilization to facilitate glucose-induced insulin secretion. It has been shown that the AT-1 antibody mutually competes for binding with OKT10 and SUN-4B7, and all three clones only bind the native (not reduced) form of CD38.