Analysis of G-protein-coupled receptor (GPCR) subtypes has attracted considerable interest because some drugs that act on GPCRs cause therapeutic problems as a result of their failure to differentiate between subtypes. In this article, an extensive compendium of diagnostic 'fingerprints' for GPCR subtypes and their families will be described. These fingerprints offer new opportunities to investigate correlations between specific sequence motifs and ligand binding or G-protein coupling, and are likely to prove valuable both in seeking novel receptors in genome data and in the characterization of orphan receptors.