> They infused into her spinal fluid trillions of viruses that carried instructions to assemble a so-called base editor customized to fix the faulty gene in her brain cells. Her condition was not likely to be fatal, but it could bring lifelong disability and serious medical issues. So her parents turned to an academic team, who developed the base editor and showed them evidence that it worked in the brains of mice and monkeys. The parents were told the risks were modest and decided treating their daughter was worth the gamble.

> The trial went tragically wrong, however, according to a Science and Retraction Watch investigation written by Brendan Borell. The child, whose parents asked that she be called Mei (Chinese for “beautiful”), developed a fever and then signs of kidney damage. She died within a week after being treated.

> The most likely reason, a hospital report would conclude, was an immune reaction to the viruses used to deliver the base editor. Mei’s death last year was never made public, and her parents—who paid for the development of the treatment themselves—now want accountability. “We did not realize how unusual and dangerous many of the arrangements were,” they say.