The study will evaluate the tolerability, safety and efficacy of gene therapy product in boys with Duchenne muscular dystrophy (DMD). In Phase I the participants will be included in two sequential dose cohorts with increasing doses of the investigational product. Based on the results of Phase I, the dose of the investigational product for use in Phase II will be determined. Phase II is a randomized, single-blind, placebo-controlled study. The participants who are randomized to the placebo arm will have an opportunity for treatment with gene therapy at the beginning of the second year.