The overall, long-term goals of this project are two fold: to understand how gene expression is regulated during mammalian skeletal muscle development and growth, and to apply that understanding to the analysis of human neuromuscular diseases. The key experimental technique to be used in this work is be the analysis of patterns of contractile protein expression in single skeletal muscle fibers by polyacrylamide gel electrophoresis. Using this procedure we have shown that skeletal muscle fibers are far more heterogeneous than either current histochemical, physiological or biochemical analyses suggest. The recognition of that heterogeneity has led us to propose a model for the regulation of gene expression in skeletal muscle which recognizes multiple types of fast and slow fibers, suggests that thick and thin filament protein synthesis is independent, and proposes that muscle cells respond rapidly to changes in their microenvironment by promiscuous expression within the multiple fast and slow programs we have defined. The implications of this model will be tested by concerted biochemical and single fiber studies on trunk, limb, craniofacial and neonatal muscles of the rabbit. In these studies the molecular bases of fiber diversity will be analyzed by characterizing the new species of myofibrillar proteins we have identified. The regulatory mechanisms that generate the diversity studied by pulse-labelling of muscle, and the effects of cell lineage and development analyzed by studies on the craniofacial muscles and neonatal muscles. The clinical implications of the model will be explored in single fiber studies of normal human muscle and biopsies from patients suffering from myotonic dystrophy nemaline myopathy and several metabolic myopathies.