Son of Sevenless 1 protein is an activator of ras proteins by stimulating the exchange of GDP for GTP on ras. Murine Son of Sevenless proteins have been shown to form a complex with activated EGF receptor, coupling tyrosine kinases to ras proteins. The SOS gene was originally isolated in Drosophila. Two mammalian homologues (mSOS1 and mSOS2) have been isolated from a mouse cDNA library and are expressed in a variety of mouse embryo and adult tissues. Human SOS1 and SOS2 homologs have also been isolated. The two mouse homologs are approximately 70% identical in composition. The mammalian SOS1 protein has a highly specific guanine nucleotide exchange activity towards p21ras. In epidermal growth factor stimulated cells, SOS1 interacts with SH3 domains of GRB2 and binds via SH2 domain to tyrosine 1068 of activated Epidermal Growth Factor receptor.