The team set to work developing a tailored treatment using base editing—a form of CRISPR that can correct genetic “misspellings” by changing single bases, the basic units of DNA. They tested it in human cells, mice, and monkeys, and KJ received an initial low dose when he was seven months old. He later received two higher doses. Today, KJ is doing well. At an event in October, his happy parents described how he was meeting all his developmental milestones.