Signal transduction molecules translate extracellular inputs into their corresponding intracellular responses. Given the complexity and number of signaling pathways present in the eukaryotic cell, it is not surprising that the functions of signaling molecules are often tightly regulated. Autoinhibition is a prevalent mechanism for governing the function of signaling molecules. The relationship between the viral, oncogenic form of Src (v-Src) and the corresponding cellular proto-oncogene (c-Src) highlights the importance of inhibitory intramolecular interactions. Src provides an example of the dramatic cellular consequences arising from the loss of autoregulation.