Cdc42 proteins are Rho GTPase family members, which act as molecular switches in regulating a variety of biological response pathways including cell motility, cell-cycle progression, gene transcription, and cell transformation. Cdc42 GTPases regulate molecular events by cycling from the inactive GDP-bound state to active GTP-bound forms. The ratio of GDP versus GTP, which is bound to the GTPase, is determined by the opposing effects of guanine nucleotide exchange factors and GTPase activating proteins. Active (GTP-bound) Cdc42 binds to the p21-binding domain of p21-activated protein kinase 1 (PAK1) to regulate downstream signaling cascades.