A kinase is an enzyme that attaches a phosphate group to a protein. Kinases serve as mobile transfer stations in a sprawling network of intracellular communications. Many of the messages they transmit tell the cell when to grow and divide. Some kinases initiate the signals; others act as receptors, receiving the messages and forwarding them. What transpires between the enzymes is an elaborate relay race, with each protein passing a “baton” of information to the next molecule in the chain.