A 4-year-old boy is brought to the physician by his parents because he falls a lot, cannot jump, and tires easily Physical examination reveals weakness in the pelvic and shoulder girdles and enlargement of the child's calf muscles. The serum level of creatine kinase is elevated. A biopsy of calf muscle reveals marked variation in size and shape of muscle fibers. There are foci of muscle fiber necrosis, myophagocytosis, regenerating fibers, and fibrosis. Molecular diagnostic assays performed on muscle biopsy from the patient would show alterations in the length of the primary transcript for which of the following muscle-associated proteins?