A number of membrane receptors coupled to GTP binding regulatory proteins have recently been shown to share structural propertie at the level both of the genes and of the corresponding proteins (Fig. 1 and 2). By comparing results obtained for α2-, β1- and β2-adrenergic catecholamine receptors, M1 to M5 muscarinic acetylcholine receptors as well as receptors for the neuropeptides substance K and angiotensin, and for dopamine and serotonin (see references in legend of figure 1), a number of common characteristics have been identified.