Researchers have for the first time identified a set of 30 inherited recessive genes that play a role in intellectual disability and possibly other brain disorders. The neurodevelopmental disorder affects as many as 213 million people around the world, according to the US Centres for Disease Control (CDC). Intellectual disability, or ID (previously known as mental retardation), becomes apparent in children before the age of 18, said researchers at the University of Maryland School of Medicine (UM SOM) in the US. The disorder, which is measured by an intelligence quotient below 70, significantly limits an individual's intellectual ability and practical skills. ID also can be a significant burden to families, society and the healthcare system. Currently, about 1-3 per cent of the world's population possess some form of ID, researchers said. While about half of all ID cases can be linked to environmental causes like poor nutrition, unknown genetic factors, such as gene mutations, are responsible for the remaining 50 per cent. The study, including researchers at Radboud University Medical Centre in the Netherlands and University of Health Sciences in Pakistan, is an outcomes of a five-year investigation that was conducted over three continents. In order to identify potential genetic causes for ID, researchers assembled a test group of 121 families in rural Pakistan, in which there was a higher incidence of ID and consanguineous marriages (marriages between blood relations). More than 15,000 DNA samples were collected, which were analysed using next-generation genetic sequencing. From an initial pool of 2,000 possible genes, the study categorised 30 novel candidate genes possessing a strong potential for causing ID - and possibly other brain disorders as well. In addition, this information now can be applied to DNA screenings in determining the possibility of a couple producing an ID child. "The implications are enormous. The next phase of our study is to come up with therapeutic options and personalised protocols that could help patients improve their intellectual function," said Saima Riazuddin, Professor at UM SOM. The research was published in the journal Molecular Psychiatry.