Reporter-gene assays offer an alternative to biochemical assays for following signal transduction pathways from receptors at the cell surface to nuclear gene transcription in living cells. Specific reporter-gene systems are now available for the study of ligand activity at G alpha(i/o), G alpha(s) and G alpha(q) G-protein-coupled receptors. In recent years reporter genes have been applied in academia and industry to the study of ligand efficacy and affinity in recombinant and primary cell lines using a variety of colour, fluorescent or luminescent read-outs.