This study is testing a new treatment, NS-089/NCNP-02, for boys aged 4 to under 15 with Duchenne Muscular Dystrophy (DMD). DMD is a muscle disorder caused by certain genetic mutations. The study is for boys whose mutations can be fixed by skipping exon 44, a part of the gene. The medicine is given through an IV (a needle in the vein) once a week. There are two parts: Part 1 with 6 boys, and Part 2 adds 14 more boys.

Boys need to be able to walk by themselves and perform a simple stand-up test in less than 7 seconds. They should have been on a stable dose of glucocorticoid, a type of medicine, for at least 3 months. Boys who weigh less than 20 kg, have heart problems, or have recently had other treatments or surgeries cannot join.

• The study lasts for two parts, with more boys joining in Part 2.
• Participants receive weekly IV infusions.
• Eligibility includes specific age, weight, and health conditions.