Hundreds or thousands of chemical and physical stimuli regulate the functions of eukaryotic
cells by controlling the activities of a surprisingly small number of core signaling units that
have been duplicated and adapted to achieve the necessary diversity. The most prevalent of
these units, at least in animal cells, are three-protein modules consisting of signal
recognition elements (receptors) and signal generators (effectors) whose activities are
linked and coordinated by heterotrimeric guanine nucleotide-binding proteins or G proteins …