The c-myc proto-oncogene was discovered as the cellular homologue of the transforming oncogene of several chicken retroviruses. It has become clear that c-myc is a member of a small gene family; members of this family encode transcription factors of the Helix-loop-Helix family of transcription factors that can both activate and repress transcription. In vivo, these genes control both proliferation and apoptosis. This review will focus on recent progress in understanding how the encoded proteins exert their biological functions.