The switch function of Ras.

Ras cycles between the active GTP-bound and the inactive GDP-bound state. Mitogenic signals activate guanine GEFs such as SOS and CDC25. GEFs increase the rate of dissociation of GDP and stabilize the nucleotide-free form of Ras, leading to binding of GTP to Ras proteins. Ras can also be activated by the inhibition of the GAPs.