Themes in Signaling   Inactive receptor monomers (green) are in equilibrium with inactive (green) or active (blue) receptor dimers. The active receptor dimers exist in a conformation compatible with trans-autop hosphorylation and stimulation of the intrinsic activity of the protein tyrosine kinase domain. Ligand binding stabilizes the formation of active receptor dimers resulting in enhancement of protein tyrosine kinase activatity.