Researchers at Cardiff University have discovered a gene which they believe is likely to be one of the causes of dyslexia in children.
People with dyslexia have difficulty with letters and numbers
It is hoped the discovery will lead to better understand of the brain disorder which disrupts reading and writing.
It is believed to be the first time a single gene linked to the condition has been identified.
The team analysed 300 families from Wales and the west of England who had at least one child with dyslexia.
They isolated the gene by comparing these results with those from families who do not suffer from the condition, which affects around 5% of the UK population.
The research will now focus on how the gene, called KIAA0319, works within the brain to disrupt reading and writing skills.
The research has been carried out by a team from the Department of Psychological Medicine at Wales College of Medicine, who described their findings as a major breakthrough.
They are looking for new volunteers to take part in their research.
Professor Julie Williams, who led the team, told BBC Wales: "We have known for a number of years there is a gene on chromosome six that confers susceptibility to develop dyslexia.
"We along with other groups throughout the world have been chasing this gene.
"We found a number of DNA variants in children we tested with dyslexia and compared them with children without dyslexia.
"We found there were a lot of differences coming from just one single gene."
She added: "There will probably be a number of genes as well as environmental factors that contribute to dyslexia.
"We need to find out how this gene functions and how it actually contributes to dyslexia.
"Perhaps it may also tell us how it contributes to the way we process language normally, which is one of the major puzzles for neuroscience today."
Actress Ruth Madoc, most famous for her role in Hi-He-Hi and more latterly in Little Britain, was diagnosed with dyslexia when she was still a girl.
She said the condition caused her problems at the start of her career because she would find herself in auditions when she had to read from a script unprepared.
She said: "I would fall down at that - it would be very difficult for me."
She said her parents "never let it be a stumbling block in any way" but she welcomed the research findings at Cardiff.
"Any knowledge would take away the stigma that dyslexia has," she said.