The behavior of eukaryotic cells, particularly those of multicel lular organisms, depends on the transmission of signals from one cell to another. Such extracellular signals can take the form of hormones, antigens, cells surface molecules, or components of the extracellular matrix and exert their effects by binding to specific receptors, usually exposed on the surface of the target cell. These transmembrane receptors possess a cytoplasmic domain that al lows communication with intracellular signaling pathways, pro viding access to the regulation of gene expression, cytoskeletal architecture,...