A major genetic study has identified 14 new childhood developmental disorders.
Scientists sequenced the genes of thousands of children from across the UK and Ireland with rare, undiagnosed conditions.
The research, in the journal Nature, was co-ordinated by the Wellcome Trust Sanger Institute near Cambridge.
Matthew Hurles, who led the study, said it "ended the odyssey for parents who'd spent years trying to find the cause of their child's condition".
Tamika Kyd, aged 10, and Caitlyn Hunter, aged nine, have a fault on the CDK13 gene, which triggered their developmental disorder.
None of their siblings is affected by the condition, which causes learning difficulties and affects their language and communication skills.
So far only 11 children have been identified with the disorder in the UK.
The families live just 20 minutes' drive from each other, north of London.
'Gives you goosebumps'
The BBC was present when they met for the first time.
The families said they were astonished at how much the two girls resembled each other.
Katja Kyd, mother of Tamika, told me: "To look at a stranger's child and see so many similarities with your own daughter gives you goosebumps - to look at them they could be twins."
The Deciphering Developmental Disorders study aims to find the genetic causes of rare childhood conditions.
Researchers screened all 20,000 genes from more than 4,000 families with an affected child.
They identified 14 new developmental disorders, all caused by spontaneous new mutations not found in the DNA of either parent.
They calculated that, on average, one in 300 children is born with a rare developmental disorder caused by a new mutation.
This adds up to 2,000 children a year in the UK and 400,000 globally.
Although Tamika and Caitlyn have the same disorder, they have some marked differences in their symptoms.
Tamika has a heart defect but Caitlyn does not.
Both have learning difficulties but Tamika's language and communication skills are more advanced than those of Caitlyn.
Vikki Thompson, Caitlyn's mother, told me: "Seeing Tamika talking so much gives me hope that Caitlyn's speech will form."
Both families were told last year that the mutation on the CDK13 gene was spontaneous and not passed on by either parent.
Tamika's mother Katja said this gave her sufficient reassurance to go ahead and try for another child.
Her newborn son Timo and her other daughter Aimee, aged nine, are unaffected.
Both women said it was hugely beneficial to be part of the research project and to be given a definitive diagnosis for their daughter's condition.
Katya said it was like "belonging to a club or a newfound family", while Vicki added: "It felt like we were on our own for the past nine years and now all that has changed."
Dr Hurles, head of human genetics at the Sanger Institute, said: "Finding a diagnosis can be a huge relief for parents and enables them to link up with other families with the same disorder."
Prof David Fitzpatrick, a supervising author from the MRC Human Genetics Unit, University of Edinburgh, said the research could "give clues for further research into future therapeutics".
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