The primary purpose of this second cohort is to see if the IV Medication, gentamicin, is safe to give twice a week for six months to boys with Duchenne muscular dystrophy (DMD). Secondarily, we want to know if gentamicin can help strengthen the muscles of boys with DMD who have a particular type of genetic mutation known as a stop codon. The gentamicin is thought to allow for "read-through" of this type of mutation which would allow for the production of dystrophin, a protein which is lacking in boys with DMD.