Growth factor receptor bound protein 2 (GRB2), also known as Ash protein, is a 24 kDa protein that contains a central Src homology (SH2) domain flanked by two SH3 domains. GRB2 is believed to be a regulatory subunit of signaling molecules whose activity is modulated by receptor binding. GRB2 associates with activated (tyrosine phosphorylated) EGFR and PDGFR via its SH2 domain as well as IRS1, SHC, and LNK through SH2 and SH3 domains. The SH3 domain binds to SOS, a guanine nucleotide exchange