The growth hormone receptor (GHR) belongs to the superfamily of transmembrane proteins that includes the prolactin receptor and a number of class 1 cytokine receptors. It exists in two forms, full-length membrane-bound receptor with a single transmembrane region, and growth hormone binding protein (GHBP), a shorter soluble form corresponding to the extracellular domain of the full-length receptor. In rodents GHBP is encoded by a specific mRNA variant while in humans it results from proteolytic cleavage of the membrane receptor by a metalloprotease. The classical view is that growth hormone sequentially binds 2 molecules of GHR inducing dimerization, but it is now widely accepted that GHR dimers exist prior to ligand binding (Poger & Mark 2010).