Small GTPases of the Ras superfamily are molecular switches which cycle between an active guanosine triphosphate (GTP)-bound and an inactive guanosine diphosphate (GDP)-bound state. They integrate signals from the cell surface to the nucleus, regulating important cellular activities. For example, Ras itself is activated when extracellular growth factors such as platelet derived growth factor (PDGF) or epidermal growth facor (EGF) bind to their receptors at the cell surface. This activation of Ras ultimately leads to changes in the transcriptional activity of the cell, e.g., via the canonical mitogen activated protein kinase (MAPK) cascade. Constitutively activated, mutant forms of Ras such as RasV12 are found frequently in human tumors, and it is widely assumed that this oncogene acts via transcriptional activation of growth and proliferation pathways.
