About 5,500 people with severe developmental disorders now know the genetic cause of their condition thanks to a major study that will be used to improve the speed and accuracy of NHS diagnoses. More than 13,500 families with a child with a severe developmental disorder across the UK and Ireland had their genomes sequenced to establish what genetic change had led to their condition, and whether it was inherited or the result of a mutation. All the children had previously been undiagnosed despite being tested. Caroline Wright, a professor of genomic medicine at the University of Exeter and the study’s lead author, said the families had been desperate for answers, which can make a huge difference in accessing support and treatment as well as quality of life. “Getting the right diagnosis is absolutely critical for families with rare conditions, which collectively affect around one in 17 people. Most of these conditions are genetic and can be diagnosed using the same genomic sequencing technology,” she said. The researchers were able to provide genetic diagnoses for 5,500 children in more than 800 different genes, including 60 new conditions previously discovered by the study, which is a collaboration between the NHS and the Wellcome Sanger Institute supported by the National Institute for Health and Care Research. About three-quarters of the conditions were caused by spontaneous mutations not inherited from either parent, according to the study published in the New England Journal of Medicine. The research team also found that the chance of diagnosis was lower in families of non-European ancestry, reinforcing the need to increase research participation for underrepresented groups. Jessica Fisher received a diagnosis for her son Mungo’s rare genetic disorder through the project in 2015, when he was 18, putting an end to years of uncertainty. She is now connected via the study with another family whose child has been diagnosed with Turnpenny-Fry syndrome, which causes learning difficulties, impaired growth and distinctive facial features including a large forehead and sparse hair, and with 36 families across the world via Facebook. “When I first saw a picture emailed to me of the other family’s child it was really emotional. We’d always looked around for children who might look like Mungo, and here was a child in Australia who could have been his sibling. It’s devastating to learn that your child has a rare genetic disorder, but getting the diagnosis has been key to bringing us together,” she said. The study was part-funded by the Department for Health and Social Care as part of a government drive to create “the most advanced genomic healthcare system in the world”. A similar approach to diagnosing individuals with rare diseases is now being used in the NHS, which will draw on the genetic conditions identified in the study to help diagnose more people swiftly. The health minister Will Quince said: “Using cutting-edge, hi-tech methods such as this offers the potential to better understand and more accurately diagnose rare genetic conditions so children can access treatment faster and potentially limit the impact of the disease on their life.” Matthew Hurles, the incoming director of the Wellcome Sanger Institute, honorary professor of human genetics at the University of Cambridge and senior co-author of the study, said the diagnoses were only possible by sharing data across all diagnostic centres in the UK and Ireland, as well as some from international colleagues. “As these genomic technologies move into routine healthcare, ensuring that undiagnosed patients can still benefit from research on their data will remain incredibly important,” he said.