Imagine being born with a gene error so serious that it destroys your body’s ability to process protein — meaning your only hope is a liver transplant, or perhaps nothing at all. That was the reality for a baby boy named KJ Muldoon, who arrived into this world with a rare, life-threatening genetic disorder called CPS1 deficiency, a metabolic condition that causes toxic ammonia to build up in the body. CBS News+2National Institutes of Health (NIH)+2
But 2025 brought a turning point. A team at Children's Hospital of Philadelphia (CHOP) and Penn Medicine used CRISPR — the gene-editing tool that has revolutionized biology — to build a personalized therapy designed specifically for KJ’s unique mutation. Penn Medicine+2National Institutes of Health (NIH)+2
🧬 What is CRISPR gene therapy
CRISPR is a technique that allows scientists to precisely edit DNA inside living cells — correcting “typos” in the genome that cause disease. In this case, researchers used a refined method called base editing, which changes a single letter of DNA code rather than cutting the DNA completely. Harvard Gazette+2Scientific American+2
For KJ, the correction was delivered directly to his liver cells using lipid nanoparticles. Once inside, the edited DNA allowed his body to process protein properly — restoring a function lost since birth. National Institutes of Health (NIH)+2Technology Networks+2
🌟 The first real-world success: a CRISPR baby
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The whole process — from diagnosis to tailor-made therapy — took just six months. National Institutes of Health (NIH)+2ABC+2
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The baby received his first dose between six and seven months of age, followed by two more doses. Penn Medicine+2ScienceDaily+2
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Weeks after treatment, he began to improve: he could tolerate more protein; needed less ammonia-lowering medication; and began hitting developmental milestones like sitting up. Live Science+2CBS News+2
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For now, he is reportedly thriving, a dramatic and hopeful turn from his earlier prognosis. Scientific American+2CBS News+2
This isn’t just a “win” for one child — it represents a milestone for medicine. For the first time, CRISPR has been used successfully as a personalized gene therapy in a living human, raising hopes that many other rare genetic diseases — once thought untreatable — could someday be fixed in the same way. PR Newswire+2Medical Xpress+2
Of course — important caveats. This is a single case. The long-term safety and durability of the gene edit remain unknown. And because the therapy was tailored specifically to KJ, scaling this kind of treatment to many patients — each with different mutations — poses huge scientific, logistical, and regulatory challenges. ABC+2STAT+2
The story of KJ shows a future where we might not only treat symptoms of genetic diseases — but correct the underlying code. CRISPR is no longer science fiction; it’s becoming real medicine.
