HRAS and RASopathy: The pathophysiology of RASopathies is centred on excessive foetal astrogliogenesis and prevalence of functionally deranged astrocytes.484,485 In Costello syndrome, which is a form of RASopathy caused by mutations of the gene encoding HRAS protein, astrocytes are hypertrophic and demonstrate increased proliferative activity; it was argued that these astrocytes increase synaptogenesis through elevated secretion of synaptogenic factors and premature formation of perineuronal net.486