The explosion in the number of cloned receptors presents the pharmaceutical industry with challenges to discover new drugs targeting those receptors; to find more-selective drugs for all novel receptor subtypes; and to learn more about the function of the receptors in order to discern the conditions where such drugs may be applied usefully as therapeutics. At the same time, receptor cloning affords an unprecedented opportunity to address these challenges: heterologously expressed recombinant human receptors can be used for drug screening and - through an improved understanding of structure-function relationship - possibly for drug design, while the receptor clones permit mobilization of the full power of molecular biology to elucidate the function of the receptors in health and disease.