In muscle cells, as in a variety of cell types, proliferation and differentiation are mutually exclusive events controlled by a balance of opposing cellular signals. Members of the MyoD family of muscle-specific helix-loop-helix proteins which, in collaboration with ubiquitous factors, activate muscle differentiation and inhibit cell proliferation function at the nexus of the cellular circuits that control proliferation and differentiation of muscle cells. The activities of these myogenic regulators are negatively regulated by peptide growth factors and activated oncogenes whose products transmit growth signals from the membrane to the nucleus. Recent studies have revealed multiple mechanisms through which intracellular growth factor signals may interfere with the functions of the myogenic regulators. When expressed at high levels, members of the MyoD family can override mitogenic signals and can cause growth arrest independent of their effects on differentiation. The ability of these myogenic regulators to inhibit proliferation of normal as well as transformed cells from multiple lineages suggests that they interact with conserved components of the cellular machinery involved in cell cycle progression and that similar types of regulatory factors participate in differentiation and cell cycle control in diverse cell types.