This is a Phase II, multiple center, 2-period, randomized, placebo-controlled, dose finding study of NS-065/NCNP-01 administered by infusion once weekly for 24 weeks to ambulant boys ages 4-<10 years with DMD. Two dose level cohorts will be enrolled. Period 1 of this study will be conducted in a double-blind fashion. Randomized patients will receive weekly IV infusions of NS-065/NCNP-01 or placebo for the first 4 weeks of their participation (Period 1) and NS-065/NCNP-01 by IV infusion for weeks 5-24 (20 weeks of active treatment - Period 2). Analysis of safety data from Period 1 of the 40mg/kg dose cohort will be completed prior to enrolling patients in the 80mg/kg dose cohort.