Activation by and desensitization to signals originating from the environment is common in all organisms, ranging from prokaryotes to higher eukaryotes. Recently, a number of novel mechanisms involved in activation and desensitization have been elucidated. The similarities of the molecular mechanisms of activation and desensitization in the various systems are striking. In the first book (1987) on this topic, the functional and structural similarities of the various mechanisms to attenuate the effect of an outward signal were emphasized. Justification to have a second book three years later...