First Baby to Undergo Gene Editing Takes First Steps: ‘It’s All Been a Miracle’

Months after becoming the first infant to receive a customized gene-editing therapy for a rare genetic disorder, 1-year-old KJ Muldoon has reached another major milestone: he’s taking his first steps.

The Children’s Hospital of Philadelphia (CHOP) shared in a Dec. 17 news release that KJ is now walking and preparing to spend Christmas at home with his parents and three siblings — a stark contrast to last year, when his first holiday season was spent in a hospital room.

“It’s all been a miracle. It’s the only way to describe it,” his father, Kyle Muldoon, told Good Morning America on Dec. 18.

KJ was born in August 2024 and was soon diagnosed with a rare metabolic disease called severe carbamoyl phosphate synthetase 1 (CPS1) deficiency. According to CHOP, he spent the first 10 months of his life hospitalized.

The condition is extremely rare, affecting about 1 in 1.3 million babies. The National Organization for Rare Disorders explains that CPS1 deficiency involves a complete or partial lack of the carbamoyl phosphate synthetase (CPS) enzyme — one of five enzymes involved in the body’s process for breaking down and removing nitrogen.

Infants with CPS1 deficiency can develop symptoms such as vomiting, refusal to eat, increasing lethargy, and, in severe cases, coma. “Unfortunately about 50% of babies pass away from this disease in the first week of life,” Dr. Rebecca Ahrens-Nicklas, director of CHOP’s Gene Therapy for Inherited Metabolic Disorders Frontier Program (GTIMD), told Good Morning America.

After KJ’s diagnosis, his parents, Kyle and Nicole, connected with Ahrens-Nicklas and Dr. Kiran Musunuru, the Barry J. Gertz Professor for Translational Research at Penn’s Perelman School of Medicine. The two physicians had begun collaborating in 2023 to explore new ways to correct genetic mutations in young children with ultra-rare diseases.

Using CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats), the team developed a custom therapy for KJ in six months. In April, CHOP reported that he received three doses without serious side effects, and by June he was discharged from the hospital after continuing to grow and do well.

“By the next day he was up laughing, looking around, playing with his toys like nothing ever happened,” Nicole told Good Morning America.

Doctors have said KJ will need lifelong monitoring to determine whether the treatment ultimately cures him, but his health has improved and his development is progressing.

Ahrens-Nicklas and Musunuru have described KJ’s early results as “quite promising,” and his case was detailed in a study published on May 15 by The New England Journal of Medicine. CHOP also noted that KJ’s case is helping drive broader conversations about how personalized therapies for rare diseases could be reviewed and approved in the future.

“We want each and every patient to have the potential to experience the same results we saw in this first patient,” Musunuru said in May, adding that he hopes other academic teams will replicate the approach for many rare diseases.

In August, KJ celebrated his first birthday at home. His parents say he’s continuing to hit key milestones — including his first steps, which often happen between 9 and 18 months for many babies. These days, he’s also joining his family in cheering on the Philadelphia Eagles.

“The day he was born, I ordered a Jalen Hurts jersey,” Kyle told Good Morning America. “I just didn’t know if he was ever gonna wear it.”

“Just to see him do these things that we never thought were possible have been inspiring,” he added.