Recent studies indicate that vertebrate skeletal muscle originates from two distinct populations of muscle precursor cells formed in early embryogenesis. Divergent patterns of expression for the myoD family of myogenic regulatory genes in different vertebrate embryos suggest some functional redundancy amongst the myogenic factors. Initial gene knockout experiments in mice support this view. Systematic mutagenesis has identified a conserved motif in the basic region of these proteins that is necessary for their biological activity. Direct interaction of both MyoD and myogenin with the transcription factor Jun indicates a mechanism for integrating the control of myoblast growth and differentiation.