Unlike EPH receptors, ephrinBs (EFNBs) do not possess intrinsic catalytic activity and thus rely on the recruitment of signaling molecules to signal. Phosphorylated ephrinB (p-EFNB) provides docking site for the SH2/SH3 domain-containing adapter protein cytoplasmic protein NCK2 (NCK2 aka GRB4) (Cowan & Henkemeyer 2001). GRB4 is able to bind to phosphotyrosines in EFNBs through their SH2 domains and 'PxxP' motifs through their SH3 domains. It has been postulated that GRB4 acts as a bridge between EFNBs and G protein-coupled receptor kinase interacting protein (GIT) 1 and Rac at synapses (Segura et al. 2007).