For many cell types, the ability to move across a solid surface is fundamental to their biological function. Certain aspects of cell locomotion, such as the protrusion of plasma membrane in lamellipodia and filopodia, are driven by the polymerization of actin cytoskeleton. The actin cytoskeleton is a dynamic filament network that is essential for cell movement during embryonic development, polarization, morphogenesis, cell division, immune system function and in the metastasis of cancer cells. To accomplish these varied cellular activities, tight spatial and temporal control is exerted over several aspects of the actin polymerization cycle, including the nucleation of new actin filaments and the elongation of existing ones. Members of the Wiskott-Aldrich Syndrome Protein (WASP) family such as N-WASP and at least three variants of SCAR/WAVE (WAVE 1, 2, 3) play a central role in regulating these processes...