Additional Study Information: The purpose of this study is to find out if eteplirsen can help people with the muscle disorder called Duchenne Muscular Dystrophy (DMD) and find out if eteplirsen is safe to take without causing too many side effects. Eteplirsen is an investigational (experimental) drug and can only be used in research studies. DMD is caused by a mutation (a change) in the gene (the part of cells that contain the instructions which tell our bodies how to grow and work) that makes dystrophin (a protein). Dystrophin is important for protecting muscles from stress and damage during activity. If a person has DMD, their body is not able to make enough working dystrophin to protect their muscles.