The role of myc oncogenes in cell growth and differentiation.

The myc oncoproteins are expressed in a wide range of normal adult and embryonic tissues. They are also found to be over-expressed in a variety of tumor types. All myc proteins are short-lived nuclear phosphoproteins thought to act as regulatory components of cell proliferation. The rapid induction of c-myc mRNA and protein following the addition of growth factors to quiescent cells, together with the short half-life of these molecules, suggests that they are sensitive and continuous indicators of external stimuli, consistent with a role in signal transduction. Furthermore, in untransformed cells, c-myc protein expression is tightly regulated, at least in part, by a mechanism of autoregulation. Deregulated expression of myc genes is a frequent observation in tumors and may lead to a cell becoming independent of one or more growth factors, with the concomitant potential for uncontrolled proliferation. Although the precise functions of the myc proteins are unknown, they all bear the hallmarks of multimeric DNA-binding proteins probably involved in the regulation of expression of specific genes.