H-Ras, otherwise known as p21Ras, a ubiquitously expressed lipid-anchored GTPase and member of the Ras superfamily of small GTPases, which acts as a molecular signal transduction on/off switch on the inner surface of the plasma membrane and endomembranes. H-Ras, N-Ras and K-Ras4A/4B are highly related RAS genes which regulate pathways involved in cell differentiation, proliferation, adhesion, migration and apoptosis and are known proto-oncogenes. Mutations in the H-Ras gene are responsible for the rare condition known as Costello syndrome characterized by facial, cardiovascular and musculoskeletal abnormalities and mental retardation.