The myc oncogenes have been implicated in the control of cell proliferation in both normal and neoplastic cells. There is increasing evidence that Myc proteins function as transcriptional regulators of other genes apparently involved in the control of cell proliferation. The effects of Myc on both gene expression and cell growth are differentially regulated by the recently described Max and delta Max proteins that can either cooperate or compete with Myc for sequence-specific DNA binding.