The myc family of proto-oncogenes encodes short-lived nuclear phosphoproteins (Myc) involved in the control of cell proliferation and differentiation. Here we discuss the evidence for Myc's involvement in normal and abnormal cell proliferation and review recent information on Max, a novel protein that forms a sequence-specific DNA-binding complex with Myc. The properties of the Myc: Max heterodimeric complex suggest a model for how Myc may function in the cell.