The results announced Wednesday are from the first human test of gene editing in the body, an attempt to permanently change someone's DNA to cure a disease — in this case, a genetic disorder called Hunter syndrome that often kills people in their teens.

In two patients who got a medium dose of the treatment, urine levels of large sugar compounds that are hallmarks of Hunter syndrome had fallen by half, on average, four months later — a possible sign the treatment is working. Two others who got a low dose have seen little change in these sugars so far.