Publisher Summary It has emerged in the past few years that the great majority of receptors for neurotransmitters belong to just two structural classes— namely, the ion channel receptor to which the nicotinic receptor belongs and the G-protein coupled receptor to which the muscarinic receptor belongs. Kimura has pointed out that spontaneous mutation rates are high and that these rates can be observed, probably, unrestrained in pieces of DNA in which selective pressures are absent, such as the untranslated introns. The more the restraint exercised by the requirement to produce functional proteins, the less will be the mutant frequency observed. It seems that the physiological significance of the subtypes of both muscarinic and nicotinic receptors must remain an open question at present. The analogous situation of the isozymes has been much more extensively studied. Some isozymes have quite distinct functions; others seem to be functionless outcomes of unselective mutations as mentioned at the beginning of this chapter. It is likely that both classes of variant will be found in neurotransmitters, but the interest for pharmacologists is that all variants offer the prospect of providing selective targets for drugs, which can affect restricted groups of synaptic junctions. An area of receptor pharmacology that has yet to be exploited is the discovery of drugs that do not interact with the transmitter binding location but that affect other locations, particularly, the interaction sites between subunits in both nicotinic and muscarinic receptors.