Part 1 (dose escalation) will evaluate the safety and tolerability of 2 doses (100 milligrams/kilogram \[mg/kg\] and 200 mg/kg) of eteplirsen in approximately 10 participants with DMD; Part 2 (dose finding and dose comparison) will evaluate the efficacy and safety of the high doses (100 mg/kg and 200 mg/kg) of eteplirsen compared with that of the 30 mg/kg dose of eteplirsen, in approximately 144 participants with genetically confirmed deletion mutations amenable to treatment by skipping exon 51.