Several recent studies provide valuable new information that expands the spectrum of human disease associated with mutations in CDC42.CDC42, an intracellular member of the Rho-family GTPases, is a key regulator of cell polarity that is highly conserved among eukaryotes (reviewed in [1]). By regulating the assembly of actin cytoskeletal structures in a temporal-spatial manner, CDC42, along with RAC and RHO A, controls cell shape and cell movement. In this way, CDC42 plays an important role in migration and directed functions, processes that are integral for embryonic development, hematopoiesis, or immune system function. CDC42 also influences other functions such as cell growth and proliferation through effects on multiple downstream cell signaling pathways, via actin-based platforms and/or direct interactions of CDC42 with other effectors.As a GTPase, CDC42 functions as an intracellular molecular switch. It cycles between two forms-an active form when bound to GTP and an inactive form when bound to GDP-subject to complex regulation by GTPase activation proteins (GAPs), guanosine exchange factors (GEFs), and guanosine dissociation inhibitors (GDIs). When activated, CDC42 undergoes a conformational change that enables it to associate with membranes and to interact with effector molecules, which in turn undergo their own conformational changes to exert downstream biochemical functions.