The cell-surface molecule CD9,a member of the transmembrane-4 superfamily,interacts with the integrin family and other membrane proteins,and is postulated to participate in cell migration and adhesion. Expression of CD9 enhances membrane fusion between muscle cells and promotes viral infection in some cells . It is often used as a mesenchymal stem cell marker . CD9 is also known as the p24 antigen besides MIC3,TSPAN29 because it is a protein of molecular weight 24 kD. The CD9 antigen appears to be a 227-amino acid molecule with 4 hydrophobic domains and 1 N-glycosylation site.