The Rho family of small GTPases, including Rho, Rac, and cdc42, act as molecular switches to regulate processes such as cell migration, adhesion, proliferation and differentiation. RhoA, RhoB and RhoC are all highly homologous but appear to have divergent biological functions. The best characterized of these proteins, RhoA, regulates acomysin contractility, cytokinesis, focal adhesion assembly and cell polarity. Mammalian Rac exists as three isoforms (Rac1, Rac2 and Rac3) that show high sequence similarity. Well-characterized Rac1 and cdc42 are ubiquitously expressed and play key signaling roles in cytoskeletal reorganization, membrane trafficking, transcriptional regulation, and cell growth and development. Phosphorylation of Rac1 at a putative Akt site (Ser71) may limit Rac1 activity through inhibition of GTP binding. Rac2 is expressed in cells of hematopoietic origin, while Rac3 is highly expressed in brain and in many other tissues. The Vav family of guanine-nucleotide exchange fac