Cellular growth, division and differentiation are all controlled by signal transduction pathways triggered mainly by proteins called "growth factors". Many growth factor receptors are membrane spanning proteins with protein tyrosine kinase activity. Binding of the ligand (growth factor) to its receptor leads to activation of the tyrosine kinase activity and the subsequent activation of a number of cytoplasmic proteins. These proteins form a "cascade" in which the activation of one molecule causes activation of the its downstream "neighbor" until the final target (usually a transcription factor) is reached.

The genes which encode many of the components of the signal transducing pathways involved in the control of cell division can be characterized as protooncogenes. Genes encoding growth factors, receptors for growth factors, cytoplasmic transducers and transcription factors have all been described as protooncogenes.

Oncogenic transformation of cells can be caused by constitutive activating these pathways in any number of ways. For example, over or inappropriate expression of a growth factor can result in inappropriate activation of a signal transducing pathway causing cells to grow and divide under conditions where they would not otherwise divide. Likewise, mutation of an intermediate component in the cascade which results in the protein getting "stuck" in its activated form results in a pathway whose output is always "on" and is independent of the original extracellular signal. This also leads to uncontrolled growth of cells.

One component of the signal transducing cascade involved growth regulation is the protein encoded for by the ras gene. (Actually, there is a family of closely related ras genes, but this fact is generally irrelevant for this discussion.)