hGH is produced by the somatotroph cell in the pituitary. hGH acts through binding to membrane receptors that belong to the cytokine receptor superfamily. Ligand binding induces receptor dimerization and activation of the receptorassociated kinase JAK2; this results in phosphorylation of the kinase itself, of the receptor, and of many cellular proteins including the Stat proteins as well as adaptors leading to the activation of the Ras/MAP kinase pathway and of the PI-3 kinase pathway.