There is accumulating evidence that GH signalling utilises a Src family tyrosine kinase independently of JAK2, and that this is linked to activation of extracellular regulated kinases (ERKs) 1 and 2 (p44/42 MAPK). The relative strengths of these signaling pathways probably depends on cell type and may be mediated by conformational changes that are a consequence of ligand binding (Rowlinson et al. 2007). In NIH-3T3 cells GH activated c-Src, which in turn activated ERK1/2 via a pathway involving the activation of the Ras-like small GTPases RalA and RalB, leading to Elk-1 mediated transcription (Zhu et al. 2002). JAK2 and c-Src were both found to activate the Ras-like small GTPases Rap1 and Rap2 which inhibit RalA mediated activation of ERK1/2 (Ling et al. 2003). Src kinase inhibition was found to block ERK activation by GH. The major contributing kinase was identified as Lyn, which was found to co-immunoprecipitate with GHR and bind to the proximal 150 residues of the cytoplasmic domain (Rowlinson et al. 2007).