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The hundreds of different protein kinases in a eukariotic cell are organized into complex networks of signaling pathways that help to coordinate the cell's activities, drive the cell cycle, and relay signals into the cell from the cell's environment. Many of the extracellular signals involved need to be both integrated and amplified by the cell. Individual protein kinases (and other signaling proteins) serve as input-output devices, or microprocessors, in the integration process. An important part of the input to these signal-processing proteins comes from the control that is exerted by phosphates added and removed from them by protein kinases and protein phosphatases, respectively.

The Src family of protein kinases (see Fig 3-10) exhibits such behavior. The Src protein was the firts tyrosin kinase to be discovered. It is now known to be part of a subfamily of nine very similar protei kinases, which are found only on multicellular animals.