The award of the 1994 Nobel Prize in Physiology or Medicine to Alfred G. Gilman and Martin Rodbell for the discovery of G (guanine nucleotide–binding) proteins and their role in cellular signal transduction has focused attention on the manifold functions of these ubiquitous molecules and on the ways in which they can become disordered in human diseases. These molecules couple a dizzying array of receptors at the cell surface (including those for neurotransmitters such as epinephrine, hormones, odorants, and photons of light) with a variety of intracellular effectors exposed at the inner surface of the plasma membrane (including enzymes such . . .