They found that the nine participants share the same genetic profile and symptoms. This amounts to a new clinical syndrome.
Each person also had the same unusual blood profile. This profile could be used to help diagnose other patients in future.
Nearly three in every 100 people have some form of learning disability and it can have many different genetic causes. Conditions also vary between patients, so each syndrome is poorly understood.
The team used human cells and a mouse model to show that our bodies need two healthy copies of the BCL11A gene to make enough BCL11A protein for our brain cells to develop normally.
The research also revealed that mutated BCL11A influences many other genes important for brain development and function. This explains why people who carry an abnormal copy have intellectual disability.
Dr Darren Logan, the lead researcher from the Wellcome Trust Sanger Institute, said: "We plan to follow this model on an even larger scale in the future, to test the role of many other genes associated with intellectual disability."