Are myc proteins transcription factors?

Expression of the myc family of cellular proto-oncogenes is critical for determining the proliferative, differentiative, and oncogenic potential of a wide variety of cell types. Despite a large body of genetic and biochemical data indicating that myc proteins are located in the nucleus and can bind to nucleic acids, the mechanism by which these proteins exert their effects remains a mystery. The recent observation that myc proteins contain two structural domains previously identified in transcription factors and differentiation factors, the leucine zipper domain and the helix-loop-helix motif, supports the notion that these proteins directly regulate gene expression. In this review we consider the possible significance of these domains to myc function.