Once recruited to the membrane in response to c-Kit stimulation, Son of sevenless (SOS1) activates the small GTPase protein RAS. SOS1 is a dual specificity guanine nucleotide exchange factor (GEF) that regulates both Ras and Rho family GTPases. SOS1 activates RAS by binding, which induces a conformational change that causes the exchange of GDP for GTP. RAS proteins are membrane-bound GTPases that regulate crucial cellular processes such as growth, proliferation and differentiation. Active RAS-GTP stimulates multiple effector proteins such as RAF-1, which induce a variety of cellular responses including initiation of mitogen activated protein kinase (MAPK) cascade signaling.