This proposal is part of a larger goal to understand myogenesis of skeletal muscle at the molecular level. Developmental processes are controlled by both tissue specific and generally expressed regulatory factors. For example, cell culture experiments have shown that both primary and immortal mouse myoblasts that express the muscle specific transcription factor MyoD but that lack tumor suppressor regulatory protein p53 activity are of limited myogenic capacity. Assays done thus far with whole mouse experiments, however, have revealed no apparent role for p53 in myogenesis. The cell culture and in vivo results together suggest a model in which p53 and MyoD play partially redundant roles. Indeed, examples of p53/MyoD redundancy are known and at least one muscle-specific gene, muscle creatine kinase, is thought to be coregulated by MyoD and p53. Preliminary results have presented additional genes that are candidates for regulation by MyoD and/or p53. Development in vivo is likely to invoke compensatory mechanisms that are unavailable to cells in culture. We hypothesize, therefore, that while failure to express either p53 or MyoD may be of less apparent consequence in the whole animal than it is in cell culture, failure to express both MyoD and p53 might result in abnormal muscle development. Analysis of p53/MyoD double knockout mice will be used to determine whether successful myogenesis requires that eitherp53 or MyoD be present. Microarray expression analysis will be used to catalog candidate genes for redundant or combinatoric regulation. The identification of activities that are regulated by either MyoD or p53 will generate testable hypotheses about how these activities affect myogenesis and about why cancer frequently results in loss of the differentiated phenotype. To the extent that MyoD and p53 are redundant, it will be important to consider relevant domains of MyoD for applications in cancer treatment.