Despite intensive research, the mechanisms by which deregulation of myc gene expression contributes to tumorigenesis are still not fully resolved and many aspects are still enigmatic. Several recent reviews, including one published in this series a few months ago, have summarized recent progress in our understanding of the biochemistry of Myc proteins [Eisenmann, Genes Dev. (2001) in press; Amati et al., Biochim. Biophys. Acta 1471 (2001) 135-145]. Also, the evidence documenting a central role of Myc proteins in human tumorigenesis has been extensively reviewed [Henriksson and Lüscher, Cancer Res. 68 (1996) 109-182]. In this article, we will argue that current progress allows us to present testable hypotheses on how Myc affects specific properties of transformed cells.