Four boys have received a gene-replacement treatment for muscular dystrophy, and it’s stoking hopes that scientists will cure this disease for good.

What: The biotech company Sarepta, in Cambridge, Massachusetts, said boys who received the gene therapy showed high levels of dystrophin, the protein that’s missing in the terrible disease.

Big deal? You bet. Muscular dystrophy is common, fatal, and currently incurable. But gene therapy might solve it. Sarepta’s stock shot up today by 50 percent.

How they did it: Dystrophin is one of the biggest genes in the human genome. Its size makes it too big to stuff inside a virus, which is needed to get the new gene into the body. The Sarepta team got over this hurdle by working with foreshortened “micro” versions of the gene instead. Others are eyeing CRISPR gene editing as the way to go.

More to come: Sarepta’s study is being carried out at Nationwide Children’s Hospital in Ohio. Two other similar studies, from Pfizer and Solid Biosciences, are also under way.