Video report by ITV News Science Correspondent Alok Jha
Two girls have become the first children to have previously unknown medical conditions diagnosed after volunteering to take part in a ground-breaking new DNA sequencing project.
Scientists from Genomics England's 100,000 Genomes Project scoured genetic codes and found the girls - Georgia Walburn-Green and Jessica Wright who are patients at Great Ormond Street Hospital in London - had gene mutations which caused their conditions.
Experts now hope that more children with rare and mystery illnesses can get treatment thanks to the diagnoses.
Now four, Georgia suffered a series of serious health problems, including physical and mental developmental delay, poor eyesight and kidney function and verbal dyspraxia, leaving her unable to talk.
But initial tests revealed nothing and doctors were unable to give her a diagnosis - until she signed up for the genome project and was told a year later a single gene mutation was the cause.
Her mother Amanda said the previous lack of answers was "one of the hardest points or our lives" but she now felt a "huge sense of relief".
Being diagnosed with GLUT1 Deficiency Syndrome, which means her brain does not get enough energy, has made a difference to Jessica "straight away", her mother Kate Palmer said.
She said she hoped the four-year-old's epilepsy and other symptoms could now be controlled through a special diet.
"More than anything the outcome of the project has taken the uncertainty out of life for us and the worry of not knowing what was wrong. It has allowed us to feel like we can take control of things and make positive changes for Jessica," she said
Professor Lyn Chitty, a clinician at Gosh and overall clinical lead for the project, said:
The girls are among around 75,000 people being recruited to the £300 million project who will have their genetic codes, or genomes, sequenced by scientists.
Some will have more than one version of their genome sequenced, making up the 100,000 total by 2017.
Health Secretary Jeremy Hunt said: "Ground-breaking outcomes like this one for Georgia and her family are incredibly promising."
In the video below, Genomics England explain Jessica's story and the 100,000 Genomes project: