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

Months after he became the first infant to undergo gene-editing therapy to treat his rare genetic disorder, 1-year-old KJ Muldoon is accomplishing another first — his first steps.

The Children’s Hospital of Philadelphia (CHOP) shared a Dec. 17 news release that KJ is walking and getting ready to spend Christmas at home with his parents and three siblings after spending his first holiday season in a hospital room in 2024.

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

After his birth in August 2024, KJ was diagnosed with a rare metabolic disease known as severe carbamoyl phosphate synthetase 1 (also known as CPS1) deficiency, and he spent the first 10 months of his life hospitalized, according to CHOP.

The metabolic condition affects 1 in 1.3 million babies and often leads to life-threatening outcomes. The National Organization for Rare Disorders states that it is characterized by a complete or partial lack of the carbamoyl phosphate synthetase (CPS) enzyme, one of five that play a role in the breakdown and removal of nitrogen from the body.

Children with CPS1 deficiency can experience symptoms including vomiting, refusal to eat, progressive lethargy and ending up in a coma.

“Unfortunately about 50% of babies pass away from this disease in the first week of life,” Dr. Rebecca Ahrens-Nicklas, director of the Gene Therapy for Inherited Metabolic Disorders Frontier Program (GTIMD) at CHOP, told GMA.

Soon after his diagnosis, KJ’s parents, Kyle and Nicole, connected with Ahrens-Nicklas, as well as Dr. Kiran Musunuru, the Barry J. Gertz Professor for Translational Research in Penn’s Perelman School of Medicine.

The two doctors began been working together in 2023, investigating new ways to correct genetic mutations in young children with ultra-rare diseases. With their research, they were able to create a custom therapy for KJ in six months using the tool CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats).

“Years and years of progress in gene editing and collaboration between researchers and clinicians made this moment possible, and while KJ is just one patient, we hope he is the first of many to benefit from a methodology that can be scaled to fit an individual patient’s needs,” Ahrens-Nicklas said in May.

In April, CHOP said that KJ had received three doses with no serious side effects, and by June, he was discharged from the hospital “after he continued to grow and thrive.”

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“By the next day he was up laughing, looking around, playing with his toys like nothing ever happened,” Nicole recalled to GMA.

Doctors have said that they will need to monitor him carefully for the rest of his life to see if he’s cured, but his condition has improved and he’s developing well.

Ahrens-Nicklas and Musunuru have noted that their results so far are “quite promising,” and KJ’s case was even detailed in a study, published on May 15 by The New England Journal of Medicine.

“Researchers are continuing to explore the causes and potential treatments for other difficult-to-treat metabolic disorders,” the hospital wrote. “KJ’s case is also prompting important discussions on how to move forward with new models for approving personalized therapies for rare diseases.”

“We want each and every patient to have the potential to experience the same results we saw in this first patient, and we hope that other academic investigators will replicate this method for many rare diseases and give many patients a fair shot at living a healthy life,” Musunuru said in May. “The promise of gene therapy that we’ve heard about for decades is coming to fruition, and it’s going to utterly transform the way we approach medicine.”

In August, KJ celebrated his first birthday at home, and his parents said he’s continuing to meet many other key developmental milestones — including his first steps, which often occur around 9 to 18 months in most babies.

KJ is even cheering on the Philadelphia Eagles with the rest of his family.

“The day he was born, I ordered a Jalen Hurts jersey,” Kyle told GMA. “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,” the proud dad added.