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The effects were that muscular dystrophin was alleviated throughout the body.

Food and Drug Administration gave the treatment— in which viruses deliver a gene for a short form of the muscle protein dystrophin into muscle tissue—a tentative “accelerated approval” limited to 4- and 5-year-olds possessing a dystrophin gene mutation.

But because the gene for the missing dystrophin protein is so large, a smaller version of the gene is used.

Because of a mutation in the gene for dystrophin, DMD patients lack functioning copies of the huge protein that serves as a shock absorber inside muscle fiber cells.

The DNA encoding dystrophin is too large to package into the AAVs widely used in gene therapy.