Background：

Adapter protein for several members of the tyrosine kinase receptor family. Involved in multiple signaling pathways mediated by Janus kinase (JAK) and receptor tyrosine kinases， including the receptors of insulin (INS)， insulin-like growth factor I (IGF1)， nerve growth factor (NGF)， brain-derived neurotrophic factor (BDNF)， glial cell line-derived neurotrophic factor (GDNF)， platelet-derived growth factor (PDGF) and fibroblast growth factors (FGFs). In growth hormone (GH) signaling， autophosphorylated (’Tyr-813’) JAK2 recruits SH2B1， which in turn is phosphorylated by JAK2 on tyrosine residues. These phosphotyrosines form potential binding sites for other signaling proteins. GH also promotes serine/threonine phosphorylation of SH2B1 and these phosphorylated residues may serve to recruit other proteins to the GHR-JAK2-SH2B1 complexes， such as RAC1. In leptin (LEP) signaling， binds to and potentiates the activation of JAK2 by globally enhancing downstream pathways. In response to leptin