The ordered growth and functioning of animal cells requires the transfer of information from one part of the cell to another. This information comes from a bewildering variety of sources, both within and outside the cell. Hormones, neurotransmitters, growth factors and extracellular metabolites interact with specific receptors on the plasma membrane or in the cytoplasm. Inside the cell, information is relayed by intracellular messengers, including cyclic nucleotides, inositol polyphosphates, diacylglycerol and calcium, and by GTP-binding proteins, protein kinases, and DNA-binding proteins....