Integrin alpha 4 (also called CD49d) is a 150 kDa protein that possesses a large extracellular domain involved in ligand binding，a single transmembrane domain，and an intracellular regulatory domain possessing multiple sites for phosphorylation. Integrin alpha 4 forms heterodimers with integrins beta 1 and beta 7. Integrin alpha 4 is expressed on leukocytes and leukocyte precursors，neural crest cells，and developing skeletal muscles and is essential for embryogenesis，hematopoiesis，and immune responses. The presence of integrin alpha 4 promotes cell migration and inhibits cell spreading and contractility. Integrin alpha 4 function has been implicated in the pathogenesis of multiple diseases including asthma，rheumatoid arthritis，Crohn's disease，ulcerative colitis，hepatitis C，and multiple sclerosis，and therefore，modulation of integrin alpha 4 function has become an important target for drug discovery.