The application of cloned genes and their protein products to molecular diagnostics has been an increasingly important area of pathology. The first gene to be identified by positional cloning was the Duchenne muscular dystrophy gene, mutations of which cause one of the most common and most devastating human inherited conditions. The identification of the responsible gene and the encoded dystrophin protein has resulted in a large series of studies concerning the other components of the membrane cytoskeleton of myofibers and their involvement in different forms of muscular dystrophy. Through the study of patients deficient in specific components of the muscle fiber, much is being learned about normal myofiber structure and function and dysfunction in disease states. A new frontier is the application of the normal genes and proteins toward patient therapeutics (gene therapy). Although highly experimental, delivery of therapeutic genes promises to become an important medical practice.