Lymphocyte Activation Laboratory Imperial Cancer Research Fund a GEF for Rac. The guanine nucleotide exchange activity 44 Lincoln’s Inn Fields of Vav is regulated by tyrosine phosphorylation: in Cos London WC2A 3PX cells Vav is tyrosine phosphorylated in response to United Kingdom coexpression of activated src family kinases, leading to an elevation in its catalytic activity as a racGEF (Crespo Rho family members include RhoA, RhoB, and RhoC, et al., 1997). Physiologically, Vav is rapidly and tran-Rac1 and Rac2, Cdc42Hs, RhoG, and TC10. These pro- siently tyrosine phosphorylated in response to antigen teins function as binary switches by cycling between receptor ligation in T cells, B cells, and mast cells. There the active GTP-bound state and the inactive GDP-bound are also protein complexes formed between Vav and the state. Three classes of regulatory proteins control the tyrosine kinase ZAP-70 or Vav and the adapter molecule guanine nucleotide binding cycle of Rho family GTPase: SH2 domain leukocyte protein of 76 kDa (SLP-76) in guanine nucleotide exchange factors (GEFs) promote activated T cells (Wange and Samelson, 1996; Wu et al., 1997) that are probably important in regulating Vav the transition from the inactive GDP-bound state to the cellular localization and catalytic activity. The critical active GTP-bound conformation; GTPase-activating proregulatory sites for Vav tyrosine phosphorylation have teins (GAPs) stimulate the inactivation; and guanine nunot been identified, and it is not proven that antigen cleotide dissociation inhibitors (GDIs) act to lock the receptors induce tyrosine phosphorylation of Vav on the