Just six months later, he received a Crispr treatment designed just for him. Muldoon has a rare disorder known as CPS1 deficiency, which causes a dangerous amount of ammonia to build up in the blood. After three doses of his custom treatment, KJ is starting to hit developmental milestones his parents never thought they’d see him reach. An important enzyme called CPS1 helps clear this toxic byproduct, but people with CPS1 deficiency lack this enzyme. She says the Crispr treatment probably turned KJ’s severe deficiency into a milder form of the disease, but he may still need to be on medication in the future.