G proteins play a central role in the mechanism of action of most hormones and neurotransmitters. They act as signal transducers between membrane receptors activated by extracellular stimuli on the one hand and intracellular effectors which control the concentrations of cytosolic messenger molecules (cAMP, cGMP, inositol phosphates, Ca2+) on the other. G proteins form a highly conserved family of membrane-associated proteins composed of alpha, beta and subunits. The alpha subunit, which is unique for each G protein, determines its biological activity and binds GDP and GTP. A number of diseases are already known to involve structural and/or quantitative changes of G proteins in plasma membranes. Interestingly, proteins encoded by some oncogenes show a high degree of homology with G proteins, which suggests that certain malignancies may be caused by alterations of transmembrane signaling.