Abstract
Numerous functions have been attributed to CD9 and other members of the transmembrane 4 (TM4) superfamily. CD9 is thought to be involved in cell proliferation, differentiation, motility and survival. It may also function as part of toxin and virus receptor complexes. Although much remains to be learned about molecular mechanisms, the molecule associates with several integrins, small G proteins, MHC class II molecules and other TM4 superfamily proteins on a given cell surface membrane. Here, we briefly discuss the CD9 displayed on stromal cells that support hematopoiesis and the potential importance of this molecule to osteoclast differentiation.