The biological basis of mental retardation is poorly understood. Mental retardation is associated with an immature morphology of synaptic spines, structures involved in neurotransmission and memory processes, suggesting that mental retardation is due to a deficiency in neuronal network formation. Recently, several genes involved in X-linked mental retardation (MRX) have been cloned. Investigation of the roles of these genes in neuronal development and function should lead to a better understanding of the cellular mechanisms underlying mental retardation. A significant number of MRX genes is directly involved in signal transduction through Rho proteins. These Rho proteins act as molecular switches which integrate extracellular and intracellular signals to regulate rearrangement of the actin cytoskeleton. Since the actin cytoskeleton mediates neuronal motility and morphogenesis, one can envision how mutations in proteins involved in Rho-dependent signaling result in mental retardation by altering neuronal network formation.