Something was very wrong with Kyle and Nicole Muldoon’s baby.

The doctors speculated. Maybe it was meningitis? Maybe sepsis?

They got an answer when KJ was only a week old. He had a rare genetic disorder, CPS1 deficiency, that affects just one in 1.3 million babies. If he survived, he would have severe mental and developmental delays and would eventually need a liver transplant. But half of all babies with the disorder die in the first week of life.

Doctors at Children’s Hospital of Philadelphia offered the Muldoons comfort care for their baby, a chance to forgo aggressive treatments in the face of a grim prognosis.

“We loved him, and we didn’t want him to be suffering,” Ms. Muldoon said. But she and her husband decided to give KJ a chance.

Instead, KJ has made medical history. The baby, now 9 ½ months old, became the first patient of any age to have a custom gene-editing treatment, according to his doctors. He received an infusion made just for him and designed to fix his precise mutation.

The investigators who led the effort to save KJ are presenting their work on Thursday at the annual meeting of the American Society of Cell & Gene Therapy, and are also publishing it in the New England Journal of Medicine.