Fusion proteins in which the N-terminus of a G protein alpha subunit is attached in frame to the C-terminal tail of a G-protein-coupled receptor have become widely used as experimental systems to explore the quantitative details of ligand stimulation of specific receptor G-protein combinations. In part, this reflects that they function as agonist-activated GTPases that behave with simple Michaelan kinetics. They have also been used to explore the effects of mutation in both receptor and G protein on information transfer, ligand regulation of posttranslational acylation, and the mechanism and potential selectivity of regulators of G-protein signaling.