In the study, the gene will be infused via peripheral arm vein so that it can reach all the muscles in the body. Six DMD subjects ages 3 months to 3 years in Cohort A, and six DMD subjects ages 4 years to age 7 years in Cohort B, will be enrolled. All subjects will receive intravenous micro-dystrophin vector (2X10e14 vg/kg in 10mL/kg). Subjects will have infusions over 1 hour in the Pediatric Intensive Care Unit (PICU) at Nationwide Children's Hospital. Before the gene therapy a muscle biopsy will be done at the screening visit. Subjects will have a second muscle biopsy to see if the gene allowed for replacement of the missing dystrophin protein at 90 days post delivery. After the gene transfer, patients will be carefully monitored for any side effects of the treatment. This will include blood and urine tests, as well as physical examination during the screening visits and on days 0, 1, 7, 14, 30, 60, 90, and 180, and at months 9, 12, 18, 24, 30 and 36 to make sure that there are no side effects from the gene injection.