ERBB2:EGFR and ERBB2:ERBB4 can directly recruit GRB2:SOS1 complex through phosphorylated C-tail tyrosines of EGFR (Y1068 and Y1086) and ERBB2 (Y1139) that serve as docking sites for GRB2 (Xie et al. 1995, Sepp-Lorenzino et al. 1996), which, again, results in SOS1-mediated guanyl-nucleotide exchange on RAS and activation of RAF and MAP kinases (Janes et al. 1994, Sepp-Lorenzino et al. 1996).