The cluster of differentiation (CD) system is a glycosyl phosphatidylinositol anchored membrane protein that belongs to the CD38 family.It is generally used in immunophynotyping. CD157 was discovered in a bone marrow stromal cell line where it facilitates pre-B-cell growth. CD157 is a bifunctional ectoenzyme that exhibits both ADP-ribosyl cyclase and cyclic ADP ribose hydrolase activities followed with CD38. It plays a role in rheumatoid arthritis (RA) due to its enhanced expression in RA-derived bone marrow stromal cell lines. Studies have shown that this protein have a role in predicted to function as a cell surface receptor and an immunoregulatory molecule.