Patients with Duchenne or Becker muscular dystrophy caused by nonsense mutations by may be eligible for this 2-week study. Before starting treatment, patients will have evaluations of muscle strength and general well being. Two muscle tissue samples will be taken by needle biopsy, under local anesthetic and sedation. Because of potential risks of hearing loss and kidney toxicity associated with gentamicin, patients will also have a hearing test and blood and urine tests for kidney function before starting treatment. (Currently, gentamicin is commonly prescribed for serious infections of the lungs, heart, and digestive and urinary tracts; adverse effects of hearing loss and kidney toxicity can occur with excessively high drug doses.)