In February, a six-month-old baby named KJ Muldoon became the first person ever to receive a CRISPR gene-editing treatment customized specifically for his unique genetic mutation, a milestone that researchers say marks a turning point in how medicine might approach the thousands of rare diseases that collectively affect 30 million Americans. Muldoon was born with a type of urea-cycle disorder that gives patients roughly a 50% chance of surviving infancy and typically requires a liver transplant; he is now a healthy 1-year-old who recently took his first steps.

The treatment's significance extends beyond one child. Scientists at UC Berkeley's Innovative Genomics Institute and the Children's Hospital of Philadelphia are now planning clinical trials that would use Muldoon's therapy as a template, tweaking the molecular "address" in the CRISPR system to target different mutations in other children with urea-cycle disorders. Last month, FDA officials Marty Makary and Vinay Prasad announced a new drug pathway designed to accelerate approvals for such personalized treatments -- a framework inspired in large part by Muldoon's case. Current gene-editing delivery mechanisms limit treatments to disorders in the blood and liver. Many families will still go without bespoke therapies.

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