ACE-031, a soluble form of the human activin receptor type IIB, was administered once every 2 to 4 weeks by subcutaneous (SC) injection to boys with DMD. Dose levels and regimens for this multiple-dose study were based on data from the initial clinical studies in healthy subjects in which doses of 0.02 to 3 mg/kg SC were evaluated. A total of 24 subjects were enrolled into the study; 18 received ACE-031 and 6 placebo. All subjects were treated for a period of 12 weeks.The pharmacodynamic effects of ACE-031 treatment were assessed by a battery of motor function test that included the 6-Minute Walk Test, the 10-Minute Walk/Run Test, the 4-Stair Climb Test and the Gower Maneuver (GW). Muscle strength was assessed by hand-held myometry and fixed system testing. Body composition (i.e., spine BMD, lean mass, and fat mass) was assessed by whole body and lumbar spine DXA scans. Pulmonary function was assessed by forced vital capacity (FVC), maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP). ACE-031 safety was evaluated through observation of the incidence and severity of adverse events.