A 4-year-old boy is brought to the physician by his parents because he tires easily. Physical examination reveals weakness in the pelvic and shoulder girdles and enlargement of the child's calf muscle. Serum levels of creatine kinase are elevated. A biopsy of calf muscle shows marked variation in size and shape of muscle fibers. There are foci of muscle fiber necrosis, with myophagocytosis, regenerating fibers, and fibrosis. Molecular diagnostic assays would most likely show alterations in the length of the primary transcript for which of the following muscle-associated proteins?