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A six-year-old British boy is the only person on Earth to suffer from a genetic disorder that leaves him unable to speak.
Doctors were left baffled when tests results showed a mutation in Mackenzie Fox-Byrne's genes that had never been seen before.
So little is known about the condition, which means Mackenzie has an extra part of a chromosome in every one of his cells, that it has not even been given a name yet.
Unique: Mackenzie Fox-Byrne, 6, is the only known person to suffer from a condition which means he has an extra half chromosome in all of his cells
His mother, Sharon, first noticed something was wrong when his development appeared to be behind that of her other children Kamara, 14, and Katie, 12.
At three months old, Mackenzie was still unable to lift his head from his cot, found it difficult to hold down food and had trouble sleeping.
Doctors first feared he may have the muscle-wasting disease muscular dystrophy, but tests results showed he had the unique disorder.
Mystery: Mackenzie is unable to speak, has learning difficulties and has no sense of danger as a result of the gene mutation
Mrs Fox-Byrne, 40, said a telephone call from Wessex Clinical Genetics Service, at Southampton University Hospital, left her stunned.
She said: 'They knew it was unique and told me excitedly that they had found something rare that no one else has.
'Unfortunately, that was all they could tell me. They couldn't tell me how he is going to progress or whether he might fall ill in the future.
'No one else on Earth has ever had this condition.'
Mackenzie's test results showed he had a triplication of a small region on the long arm on his X-chromosome.
At the moment, he is unable to speak, has low muscle tone, is still in nappies and has no sense of danger.
He also has learning difficulties which means he as the mental age of a two-year-old and goes to a special school in Shrewsbury.
Mrs Fox-Byrne, who lives with her partner Andy, 47, in Market Drayton, Shropshire, said: 'It's quite terrifying to be told he is the only person in the world to have this condition.
'But for all his problems Mackenzie is a lovely boy. He's always giving me kisses and cuddles and likes to sleep next to me because he still has trouble sleeping.
'Although we worry about what might happen to him in the future, I just try to put it out of my mind. You could go crazy thinking about it.
'It would be fantastic to actually know what he had because at the moment we don't know what his life expectancy will be or whether he might become ill in the future.
'We do know he can never be allowed to have children as he would pass his genes on to them and we wouldn't know what their condition might turn out to be.
'My hope for him is that someday he will get better and be able to live a normal life like other children, but I know the likelihood of that happening is very small.'
Karen Temple, professor of medical genetics at Wessex Clinical Genetics Service, confirmed Mackenzie was a totally unique case.
She said: 'In this case there is a rearranging of the chromosomes that is unique to him. We have to learn what we can from the little boy as he grows up.
'In every cell Mackenzie has got an extra part of a chromosome. In layman's terms if you think of some beads as part of a chromosome, he has got two extra parts.
'This little boy has had this chromosome problem since he was conceived, we can learn from how he is now and that helps us to predict his future.'
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