SOS1 is the guanine nucleotide exchange factor (GEF) for RAS. SOS1 activates RAS nucleotide exchange from the inactive form (bound to GDP) to an active form (bound to GTP) (Chardin et al. 1993).
Van Aelst, L
guanyl-nucleotide exchange factor activity of GRB2:SOS1:HB-EGF:p-6Y-EGFR [plasma membrane]