BOTHELL, Wash. ( TheStreet) -- Sarepta Therapeutics ( SRPT) unveiled the first data demonstrating that an experimental "genetic fix" can significantly improve the walking performance of patients with muscular dystrophy. Results from the small study are still preliminary but the Sarepta drug, known as eteplirsen, is a potential medical breakthrough because it may be the first to treat the underlying cause of Duchenne muscular dystrophy (DMD), a rare genetic disease for which there is no effective treatment. Last April, Sarepta presented data showing that eteplirsen increases levels of a key protein involved in muscle function but Tuesday's results were the first to show a more tangible, or real-world, benefit measured by an improvement in the distance a patient can walk over six minutes. Sarepta shares more than doubled in value to $8.58 after the new eteplirsen data were released. The stock's rapid rise can be attributed not only to the promising eteplirsen data but also because Sarepta may be able to seek FDA approval without having to conduct another clinical trial under new regulations designed to speed the development and approval of drugs for rare diseases. Sarepta recently changed its corporate name from AVI Biopharma. In the study, DMD patients treated weekly with 50 mg of eteplirsen and followed for 36 weeks demonstrated a decline of 8.7 meters in a six-minute walk test compared to baseline. Patients treated with a placebo for 24 weeks followed by eteplirsen for 12 weeks -- the study's control arm -- took the same walking test but showed a decline of 78 meters from baseline. The treatment benefit favoring eteplirsen as measured by walking distance was 69.4 meters over 36 weeks and was statistically significant, Sarepta said. "The magnitude of this clinical benefit is an unprecedented treatment effect in DMD. This result represents a major advance in the pursuit of a disease modifying treatment for this severe, progressive and life-threatening disease," said Dr. Jerry Mendell, Director of the Centers for Gene Therapy and Muscular Dystrophy at Nationwide Children's Hospital and principal investigator of the Phase IIb study, in a statement. DMD is a rare disease and as such, Sarepta enrolled a very small number of patients in this initial study. Just four patients were treated with the 50 mg dose of eteplirsen compared against four patients treated with a placebo and delayed eteplirsen.