A signal transducing adaptor protein that links extracellular signals to the map kinase signaling system. Grb2 associates with activated epidermal growth factor RECEPTOR and platelet-derived growth factor RECEPTORS via its SH2 DOMAIN. It also binds to and translocates the son of sevenless proteins through its SH3 DOMAINS to activate PROTO-ONCOGENE PROTEIN P21(RAS).