p120GAP is a negative regulator of Ras that stimulates hydrolysis of bound GTP to GDP. Once the Ras regulator p120GAP, a member of the GAP protein family, is recruited to the membrane, it is transiently immobilized to interact with Ras-GTP. The down-regulation of Ras by p120GAP is a critical step in the regulation of many cellular processes, which is disrupted in approximately 30% of human cancers. p120GAP contains SH2, SH3, PH, calcium- and lipid-binding domains, suggesting its involvement in a complex network of cellular interactions in vivo.