Cas ligand with multiple Src homology (SH) 3 domains (CMS) is an ubiquitously expressed signal transduction molecule that interacts with the focal adhesion protein p130^Cas. CMS contains three SH3 in its NH₂ terminus and proline-rich sequences in its center region. The latter sequences mediate the binding to the SH3 domains of p130^Cas, Src-family kinases, p85 subunit of phosphatidylinositol 3-kinase, and Grb2. The COOH-terminal region contains putative actin binding sites and a coiled-coil domain that mediates homodimerization of CMS. CMS is a cytoplasmic protein that colocalizes with F-actin and p130^Cas to membrane ruffles and leading edges of cells. Ectopic expression of CMS in COS-7 cells resulted in alteration in arrangement of the actin cytoskeleton. We observed a diffuse distribution of actin in small dots and less actin fiber formation. Altogether, these features suggest that CMS functions as a scaffolding molecule with a specialized role in regulation of the actin cytoskeleton.