Now, a new startup called Aurora Therapeutics, cofounded by gene-editing pioneer Jennifer Doudna, is aiming to scale such treatments to many more patients with rare diseases. Crispr works by using a guide RNA to deliver an editing molecule to a desired location in the genome. The guide RNA is like a car’s GPS—it goes where it’s programmed to go. In the case of baby KJ, scientists built a guide RNA to target his specific genetic mutation. Aurora’s strategy involves swapping out that guide RNA to make several versions of a PKU therapy that address different mutations.