On 24 March 2025, physicians and scientists in China tried to help a 6-year-old girl whose brain wasn’t developing properly because of a genetic mutation. 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.
Multiple gene therapy experts and bioethicists who reviewed the case question whether the parents were properly informed of the therapy’s risks. “This never should have gone to trial,” one says. Questions have also been raised about whether the therapy worked as well in animals as a paper in Nature earlier this year suggests, whether its authors should have been more transparent about Mei’s death and about a primate study that raised safety concerns, and whether the journal bears any responsibility to update or retract the paper.
Here are key takeaways from our investigation.
China still struggles with biomedical oversight and transparency
In 2018, Chinese biomedical researcher He Jiankui shocked the world by announcing he had secretly edited human embryos and produced two babies he hoped would be immune to HIV. The effort was considered risky and unethical and widely condemned—even by the scientist who went on to develop Mei’s treatment. China sent He to jail and toughened its rules regulating gene editing and other biomedical treatments.
But because the country wants to rival the United States as a biotech power, China recently changed its system to speed research into human therapies. Mei’s death and the silence that followed suggest the country still has work to do, says Joy Zhang, a sociologist at the University of Kent who has written about the culture of secrecy in Chinese scientific institutions. The lack of disclosure could also fuel recent calls by politicians to ban the U.S. from considering clinical trial data from China.
Gene therapies remain risky and need careful scrutiny
Mei’s family wants to make the details of her death public in part to help protect other gene therapy patients from similar harm. Ever since a teenage boy named Jesse Gelsinger died from a gene therapy attempt in 1999, researchers have known the strategy carries significant risks.
The gene therapy field has slowly rebounded after the Gelsinger tragedy, and the ability to edit existing genes, not just add a new gene, has boosted optimism. But delivering genes and gene editors remains fraught. Other patients have also died or had serious medical issues after therapy attempts, often because their immune system overreacts to the flood of viruses typically used to deliver a gene into cells. Investigators are now trying to develop nonviral “vectors” for delivering DNA into cells.
Targeting a gene therapy to the brain, as in Mei’s case, adds to the challenge, as it requires smuggling the treatment across the blood-brain barrier. Given the risks, gene therapy researchers and ethicists say using the approach to treat nonfatal conditions is a difficult, complex decision. “I don’t think these kinds of experiments should stop, but we need to make sure we are appropriately careful about them,” says bioethicist Hank Greely, director of the Center for Law and the Biosciences at Stanford University. “It’s so easy to be blinded by hope—whether it’s hope for your kid, hope for your research, or hope for your company.”
The exorbitant cost of gene therapies can burden families
The gene therapy field is struggling with how to make the treatments affordable, especially if they are customized for individual patients, like the personalized gene editor designed for Mei, or developed for rare diseases. Because companies often don’t see a way to profit from gene therapies, several efforts have recently been launched to try to find alternative development paths. But desperate families have often resorted to financing therapies with their own money or through fundraising. Mei’s parents scraped together more than $800,000 to pay the team that developed her gene-editing treatment, under informal arrangements the father found troubling. Its development and trial would likely have cost far more in the U.S. “If the study had been done in North America, I estimate costs at $1 million for what was published in Nature, and $2 million for a n-of-1 clinical trial,” says Philippe Campeau, a medical geneticist at the University of Montréal. After Mei died, one of the Chinese scientists returned more than $100,000 the parents had paid to him
Trial death raises questions about how preclinical findings were published
Nearly a year after Mei’s death, the academic team that developed her therapy published a paper in Nature presenting evidence that it worked in mice and monkeys. Although a draft of the paper referred to the family’s genetics and acknowledged the parents’ financial support of the research, those details had been stripped from the final paper. The parents had asked the team to withdraw the paper before publication and sent a letter of protest to Nature after it came out, mentioning their financial support of the trial and their daughter’s death. The journal told them the ethical issues “fall outside our purview in terms of data integrity” and that the matter should be handled by the university. In response to our questions, an editor at Nature said that during the paper’s review process the journal was not aware of Mei’s death or related issues, such as the sanctions the hospital received for insufficient oversight.