Two children have become the first to have their undiagnosed diseases identified through the 100,000 Genomes Project.
The project, which aims to sequence 100,000 genomes from people with rare conditions and cancer, achieved a world first last year when three British men had their rare diseases diagnosed.
Now Jessica Wright and Georgia Walburn-Green, both four, have had the precise genetic origins of their previously undiagnosed conditions explained.
The pair underwent tests at Great Ormond Street Hospital, which is one of 13 "genomic medicine centres" in England that are recruiting patients with rare disorders.
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Following the tests, Georgia was identified as having a genetic abnormality in the KDM5b protein coding gene.
Jessica was also found to have a GLUT1 Deficiency Syndrome caused by a genetic error, which means her brain doesn't have enough energy to function properly.
Both Georgia and Jessica were part of the first wave of families to be recruited as part of the pilot phase of the 100,000 Genomes Project.
The results mean that both girls can be better treated in the future.
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Jessica's mother, Kate, said: "It’s the not knowing that I found really difficult. Most children with an undiagnosed condition have developmental delay, epilepsy and a squint – all of the things that Jessica has – so if you Google that you get umpteen possibilities.
"We spent hundreds of hours researching online and wondering ‘is it this terrible syndrome; is it that terrible syndrome?’
"Now that we have this diagnosis, there are things that we can do differently almost straight away.
"Her condition is one that has a high chance of improvement on a special diet, which means that her medication dose is likely to decrease and her epilepsy may be more easily controlled."
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Other children with undiagnosed disorders also stand to benefit from the pilot's success by being tested for the newly discovered genetic mutations early in their lives.
The government-led 100,000 Genomes Project was announced by the Prime Minister in 2012.
The NHS anticipates that around 70,000 people will be involved in the initiative, which will collect DNA samples that are then sent to Genomics, a Department of Health company, to be sequenced.
Results are then returned to the NHS to inform decisions on treatment.