SOS1 promotes the exchange of Ras-bound GDP by GTP. EPS8, E3B1 and SOS1 form a tricomplex in vivo that exhibits RAC-specific guanine nucleotide exchange factor activity in vitro. Defects in SOS1 are the cause of gingival fibromatosis 1 (GGF1). Defects in SOS1 are the cause of Noonan syndrome type 4 (NS4). The antibody is specific to SOS1.