Duchenne muscular dystrophy is a genetic disorder that affects about 15,000 patients, mostly boys, in the U.S. The disease can cause movement, heart and breathing problems and those with it typically lose the ability to walk, and die in their 20s or 30s. Deflazacort, which will be sold in the U.S. under the brand name Emflaza, is a steroid meant to improve muscle function, so those with the disease might be able to feed themselves, control their wheelchairs or walk longer.