Activation of Ras protein, illustration. An inactive Ras protein (left) has a GEF (guanine nucleotide exchange factor) protein bound. GEF stimulates the release of GDP (guanine diphosphate, orange) from Ras's binding site. This allows a GTP molecule (guanine triphosphate, orange) to bind to RAS, changing it into the active form (right). Ras proteins are involved in transmitting signals within cells, turning on genes involved in cell growth, differentiation and survival. Mutations in ras genes can lead to permanently activated proteins causing cells to subdivide without control, often leading to cancer.