Biochemical and genetic observations have supported the hypothesis that Myc family proteins function to regulate genes important in cellular growth and differentiation. The recent findings that Myc proteins can associate with other cellular proteins, possess sequence-specific DNA-binding activity and may directly transactivate transcription of several candidate genes have provided an experimental framework in which to test the transcription factor model. Based on principles established for several well characterized viral oncoproteins, a model is presented in which the regulation of Myc function is controlled by specific cellular protein interactions that serve to activate or repress transactivation activity or deny access of the Myc complex to its target sequences.