PHILADELPHIA — KJ Muldoon, an infant with a life-threatening metabolic disorder who received a pioneering gene-editing therapy at Children’s Hospital of Philadelphia, went home Tuesday after 307 days at the hospital, CHOP announced.

KJ risked irreparable brain damage from his condition, called severe carbamoyl phosphate synthetase 1 (CPS1) deficiency, in which his liver could not process protein. CPS1 deficiency is deadly in more than half of cases.

Doctors at CHOP rapidly developed within six months an experimental, customized treatment using the gene-editing technology CRISPR. They analyzed KJ’s genetic profile to locate the gene mutation that prevented his body from producing an enzyme that breaks down protein.

Then they administered a medication, laced with bits of genetic code, to fix the mutation.

Last month, KJ’s case was detailed in the New England Journal of Medicine. Outside experts called it a milestone — a demonstration that CRISPR technology can be used to quickly produce tailor-made treatments for patients with rare diseases.

In a video released by CHOP on Tuesday, nurses dressed KJ in a graduation gown and cap to celebrate his “graduation” from the hospital.

Employees lined the hall to applaud as his family walked him off his hospital unit in a Phillies ball cap and onesie and into a waiting car. A motorcycle unit of Philadelphia police officers escorted the family home to Clifton Heights, just outside Philadelphia.

The hospital said that the baby’s family and doctors would not be available for additional interviews.

In an interview last month, doctors said early signs indicate the CRISPR therapy has not cured KJ, but has made the effects of his disease milder. He had increased protein intake and decreased his medical doses, and his long-term medical needs remained unknown.

Still, his parents told The Inquirer last month, they were eager to bring him home to be with his three older siblings and were astonished by the progress he had already made.

“It’s been an incredible transformation,” said his father, Kyle Muldoon.

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—Staff writer Sarah Gantz contributed to this article.

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