CD157, also known as bone marrow stromal cell antigen 1 (BST-1), is a glycosyl phosphatidylinositol anchored membrane protein that belongs to the CD38 family. CD157 was discovered in a bone marrow stromal cell line where it facilitates pre-B-cell growth. Along with CD38, CD157 is a bifunctional ectoenzyme that exhibits both ADP-ribosyl cyclase and cyclic ADP ribose hydrolase activities. CD157 may play a role in rheumatoid arthritis (RA) due to its enhanced expression in RA-derived bone marrow stromal cell lines. CD157 has been predicted to function as a cell surface receptor and an immunoregulatory molecule.