The Wiskott-Aldrich syndrome (WAS) family of proteins share similar domain structure, and are involved in transduction of signals from receptors on the cell surface to the actin cytoskeleton. The presence of a number of different motifs suggests that they are regulated by a number of different stimuli, and interact with multiple proteins. Recent studies have demonstrated that these proteins, directly or indirectly, associate with the small GTPase, Cdc42, known to regulate formation of actin filaments, and the cytoskeletal organizing complex, Arp2/3, which can nucleate actin polymerization at sites that lead to branched actin structures. These proteins have 48% identity in human with the highest homology in the functional regions of these proteins. Serine and tyrosine phosphorylation regulates the activity of both proteins. WASP is observed as a 63kD protein in hematopoietic cells, while N-WASP is observed as a 65kD in many tissues, especially brain. Wiskott-Aldrich syndrome is a rare, inherited, X-linked, recessive disease characterized by immune dysregulation and microthrombocytopenia, and is caused by mutations in the WAS gene. The WAS gene product is a cytoplasmic protein, expressed exclusively in hematopoietic cells, which show signalling and cytoskeletal abnormalities in WAS patients. These clinical findings as well as the structural properties of WASP (including a cdc42 binding site, SH3 domain binding region and regions for actin cytoskeletal localization) suggest a pivotal role for WASP in regulating the structure and function of platelets and T-lymphocytes. A transcript variant arising as a result of alternative promoter usage, and containing a different 5' UTR sequence, has been described, however, its full-length nature is not known.