To help, her group has constructed detailed decision trees and leads families through them. Ms. Miller’s own son, who is 22, does not qualify for the new clinical trials.

Ryan and Brooke Saalman know how hard it can be to know what to do. “We did a lot of praying,” said Ms. Saalman, mother of two boys with Duchenne in Columbus, Ga.

They decided to enroll their oldest son, Jacob, 6, in a trial of a highly experimental drug. The trial requires weekly infusions at a site 100 miles from their home. He is one of the first boys to be treated.

The investigators are testing a strategy called exon skipping: putting a molecular bandage over a tiny mutation in a large gene. The Saalmans also considered a gene-therapy trial, in which scientists were attempting to insert a new gene that makes dystrophin.

But they discovered that gene therapy may be irreversible. And if it didn’t work, Jacob would be ineligible for an even more promising approach in the future: gene editing, to snip out the deadly mutation that causes Duchenne, an effort now in preclinical development.

Gene therapy and gene editing both depend on a disabled cold virus to deliver the treatment. Once exposed to that virus, the body makes antibodies to it. Essentially, patients like Jacob get one shot at genetic modification.