The growth hormone receptor forms dimers (Cunningham et al. 1991) that subsequently signal via JAK2 (Argetsinger et al. 1993). Early studies suggested that dimerization occured following ligand binding, but it is now generally accepted that dimerization is independent of ligand binding and involves the transmembrane and juxtamembrane domains (van den Eijnden et al. 2006, Brown et al. 2005).