p21 RAS:GTP p21 RAS:GDP SOS1-mediated nucleotide exchange of RAS (HB-EFG-initiated) Authored: Jassal, B, Tripathi, S, 2012-04-04 Edited: Jassal, B, Tripathi, S, 2012-04-04 Pubmed8493579 Reactome Database ID Release 432179407 Reactome, http://www.reactome.org ReactomeREACT_121377 Reviewed: D'Eustachio, P, 2012-04-23 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).