Children with rare mystery illnesses have been diagnosed for the first time, thanks to a ground-breaking DNA sequencing scheme.
Georgia Walburn-Green and Jessica Wright are the first children to have previously unknown conditions diagnosed after volunteering for the 100,000 Genomes Project, Genomics England said.
Scientists scoured genetic codes and found the girls, both patients at Great Ormond Street Hospital (GOSH), had gene mutations which caused their conditions.
Experts hope the diagnoses can be used to help others with the same unidentified illnesses get treatment.
Georgia, now four, 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 showed 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".
Health Secretary Jeremy Hunt said: "Ground-breaking outcomes like this one for Georgia and her family are incredibly promising, and are the reason that this government has invested over £200 million in bringing such revolutionary science to the NHS.
"By sequencing DNA on an unprecedented scale, we are also cementing the UK's position as a world leader in 21st-century medicine."
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.
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.
The first adults were diagnosed early last year.
Professor Lyn Chitty, a clinician at Gosh and overall clinical lead for the project, said: "It's really exciting to see the results coming through and the difference this can make for families. It increases confidence in the project and shows that the application of genomics can live up to the promise of changing the way we diagnose and treat patients in the future."