G protein-coupled receptors (GPCRs) are among the most important targets for pharmacological compounds used in humans. During the last 10 years, more than 1000 genes coding for GPCRs, representing approx 0.1% of the human genome, have been identified, of which at least 200 have been demonstrated to correspond to active receptors. Cloning of receptor cDNAs allowed expression in heterologous cell systems. Among the available expression systems, considerable interest has focused on microorganisms, such as Escherichia coli (E. coli), that provide a reproducible and inexpensive source of receptor, combined with ease of use, low background binding, and the absence of endogenous GPCR. All these characteristics are prerequisites for the successful application of GPCRs in high-throughput screening assays for the discovery of new drugs (1).