SOS promotes the formation of GTP-bound RAS, thus activating this protein. RAS activation results in activation of the protein kinases RAF1, B-Raf, and MAP-ERK kinase kinase (MEKK), and the catalytic subunit of PI3K, as well as of a series of RALGEFs. The activation cycle of RAS GTPases is regulated by their interaction with specific guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). GEFs promote activation by inducing the release of GDP, whereas GAPs inactivate RAS-like proteins by stimulating their intrinsic GTPase activity.