The primary objective of this study is to assess the ongoing efficacy, safety, and tolerability of an additional 212 weeks of treatment with eteplirsen injection in Duchenne muscular dystrophy (DMD) subjects who have successfully completed the 28 week eteplirsen study: Study 4658-us-201. This study will also evaluate the correlation between biomarkers for DMD and the clinical status of participating DMD subjects.

This is an open label, multiple dose extension study to assess the ongoing efficacy, safety, and tolerability of weekly intravenous (IV) infusions of eteplirsen in DMD subjects who have successfully completed Study 4658-us 201.

Subjects will have the opportunity to enroll in this study during the last visit of Study 4658-us-201 (Week 28). Eligible subjects will receive once weekly IV infusions of eteplirsen (50 or 30 mg/kg) for an additional 212 weeks. Subjects will receive the same dose of eteplirsen they received in Study 4658-us-201. Subjects will thereafter continue to receive once weekly IV infusions of eteplirsen for up to an additional 72 week period (through week 284). If commercial eteplirsen becomes available during this additional 72 week period, participation in the study will be discontinued as subjects transition to commercial eteplirsen.

Safety, efficacy, pharmacokinetic (PK), and biomarker assessments will be performed at scheduled visits; adverse events (AEs) and concomitant medications and therapies will be continuously monitored.

If review of data from this open label study suggests that continued treatment with eteplirsen is warranted, this study may be extended by protocol amendment or subjects who successfully complete this study may have the opportunity to participate in a separate follow on, open label eteplirsen study.

Inclusion Criteria:

A subject must meet all of the following criteria to be eligible for this study.

1. The subject and/or their parent/legal guardian are willing and able to provide signed informed consent.
2. The subject has successfully completed 28 weeks of treatment in Study 4658-US-201.
3. The subject has a parent(s) or legal guardian(s) who is able to understand and comply with all of the study procedure requirements.

Exclusion Criteria:

A subject who meets any of the following criteria will be excluded from this study.

1. The subject has a prior or ongoing medical condition that, in the Investigator's opinion, could adversely affect the safety of the subject or make it unlikely that the course of treatment or follow-up would be completed or impair the assessment of study results.