WITHIN DAYS after KJ was born in Philadelphia in August 2024 it was clear that something was wrong. He was not eating and slept too much. Blood tests revealed sky-high levels of ammonia, a toxic substance the body usually expels. Genome sequencing confirmed that he had a rare genetic disease called carbamoyl-phosphate synthetase 1 (CPS1) deficiency, which often kills in infancy, and for which no good neonatal treatment exists. Then one of his doctors suggested something radical: a gene-editing drug designed specifically for him.
