The Wiskott-Aldrich syndrome (WAS) is characterized by thrombocytopenia, eczema, defects in cell-mediated and humoral immunity and a propensity for lymphoproliferative diseases. The syndrome is the result of a mutation in the gene encoding a proline-rich protein termed WASP. WASP is a downstream effector of Cdc42 and has been implicated in actin polymerization and cyto- skeletal organization. Distantly related proteins, VASP (vasodilator-stimulated phosphoprotein) and Mena (for mammalian enabled protein), are involved in the regulation of cytoskeletal dynamics. Both Mena and VASP accumulate at focal adhesions. Mena is highly expressed in the developing nervous system and may be involved in growth cone motility and axon guidance.