There are three contact sites between IL-6, IL-6R and gp130, named site I, II and III. Site I and II correspond to the respective sites of the growth hormone receptor complex whereas site III is only found in receptor complexes with at least three subunits. Various models of the IL-6 receptor complex have been proposed (Scheller & Rose-John 2006), but crystallographic data suggests the assembly of a hexameric complex containing two IL-6, two IL-6RA and two gp130 subunits. The quaternary structures of other IL-6/IL-12 family signaling complexes suggest they have a similar topology (Boulanger et al. 2003).