Link. Heartbreak still likely, but “In Sarepta’s case, eteplirsen turns off just one part of the dystrophin gene, known as exon 51. In the patients who are candidates for the drug, it is exon 51 that contains the mutation that causes production of dystrophin to stop. By causing the muscle cells to skip over exon 51, eteplirsen allows them to produce a form of dystrophin that is not perfect but appears to be somewhat functional.”