Major cancer therapeutic approaches are based on inhibition of the ras-signaling pathway, with special emphasis on the MAPK arm. Transformation progression from benign to malignant can be effected by the expression of Rho GTPases, also ras effectors. To ascertain whether their inhibition, could suppress progression, dominant negative (DN) GTPases were transfected into malignantly transformed epithelial cells. N17rac gave rise to cells that, though viable, were severely depressed in their growth rate and saturation density, due to increased apoptosis. This was in contrast to cells expressing WTrac1 or the other DN GTPases, which did not exhibit altered growth kinetics. WTrac1 and N17rac transfectants were no longer able to grow in soft agar, unlike the other DN GTPase transfectants, which retained their ability to grow in soft agar. Thus, not only progression, but transformation per se was suppressed by DNrac1. V12rac1 alters the expression and localization of the actin regulating proteins vinculin and VASP, which results in the loss of stable F-actin structures and actin-based differentiation characteristics. In the presence of N17rac1, VASP was downregulated and vinculin and F-actin colocalization restored. Consequently, F-actin structures and their dependent adhesive interactions were reestablished. Thus, rac1 and its effectors may also serve as important targets for cancer therapeutics.