The Baby Who Changed Medicine

KJ Muldoon was only five months old when doctors at the Children’s Hospital of Philadelphia placed him on a liver transplant list.

He had been diagnosed with CPS1 deficiency, a rare and often life-threatening genetic disorder. Children with the condition cannot properly break down ammonia, a substance the body produces during normal digestion. Without a working liver enzyme to remove it, ammonia can build up in the blood and damage the brain.

For infants, the disease can be devastating. About half of babies with CPS1 deficiency do not survive.

KJ’s doctors had another idea.

Working with researchers at Penn Medicine, the medical team designed a personalized CRISPR gene-editing therapy made specifically for KJ’s genetic mutation. It was not a treatment pulled off the shelf. It was built for him.

The therapy used lipid nanoparticles — tiny fat-like particles — to carry the gene-editing tool directly into KJ’s liver cells. The goal was to correct the faulty enzyme so his body could better process ammonia.

The process moved with remarkable speed. From diagnosis to first treatment, it took about six months. KJ received his first infusion in late February 2025, followed by additional doses in March and April.

Today, he is walking, talking, and thriving.

The medical significance is enormous. KJ is believed to be the first person to receive a CRISPR therapy designed specifically for his individual genetic mutation. That makes his case more than a personal victory. It points toward a possible new future for treating rare genetic diseases.

Thousands of rare disorders are caused by specific genetic mistakes. For many families, those diagnoses come with few options and little hope. A treatment model that can be designed for one child’s mutation could one day be adapted for others.

There are still many questions ahead. Personalized gene editing will need careful testing, long-term monitoring, and a way to make treatment available beyond a single extraordinary case. Cost, access, and safety will all matter.

But KJ’s story shows what is now possible.

A baby once headed for a liver transplant received a treatment made just for him — and helped open a door for countless others.