This study aims to assess the safety, tolerability, pharmacokinetics and pharmacodynamics of the novel anti-myostatin adnectin RG6206 (BMS-986089) in a Phase 1b/2 study in ambulatory boys with Duchenne muscular dystrophy (DMD; NCT02515669). The primary endpoint is safety over 24 weeks. Secondary endpoints included pharmacokinetics of RG6206, frequency of anti-drug antibodies and pharmacodynamic effect on serum myostatin levels. Muscle volume and lean body mass were assessed by DXA/MRI imaging.