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Rhys Evans was born with no functioning immune system and no resistance to infection.

Even simple germs like a cold could have killed him.

Today he is celebrating his 10th birthday and he is a healthy, happy schoolboy.

Thanks to ground-breaking gene therapy treatment at Great Ormond Street Hospital, Rhys has a working immune system and is able to live a normal life.

But it was not always that way. Rhys, from the village of Nelson in Caerphilly county, was born with a fatal genetic disorder called X-SCID - X-linked severe combined immunodeficiency.

He suffered recurrent chest infections from the age of four months and was eventually admitted to hospital with pneumonia.

Doctors finally diagnosed X-SCID, a very rare disease which affects only two or three children in the UK each year.

In 2001 Rhys began his gene therapy treatment.

Nine other boys with the same disorder have now been treated in the same way at the London hospital.

"Most of the children have recovered good immunity," says Professor Adrian Thrasher, consultant immunologist at Great Ormond Street Hospital.

The disease is often known as 'baby in the bubble syndrome', because affected babies have to be kept well away from any infections.

Rhys's mother Marie Evans explains: "Before Rhys's treatment he was cut off from the outside world in a super-clinical room where he could only be visited under strict supervision.

"He was so weak he couldn't even hold his head up," she said.

New instructions

During gene therapy, a working copy of the defective gene is placed in the child's own bone marrow cells using a virus and these are then returned to the child's body.

The hope is that the modified cells grow and help restore the child's immune system.

Without the treatment, Rhys would have died before his second birthday.

In 2007, however, Great Ormond Street hospital announced that one of the 10 children receiving gene therapy had developed leukaemia.

Subsequently, there were headlines questioning the treatment programme.

But leukaemia was "an acknowledged risk" of the treatment for this specific group of diseases, says Professor Thrasher, which they were prepared for.

It is thought the implanted gene was planted next to, and switched on, a gene that causes cancer.

Other similar studies in France and the US had also experienced the development of leukaemia in a small number of patients.

"We have learnt why that's happened," says Prof Thrasher. "We've now redesigned the vector which carries the gene into the cells and the risks are much, much lower."

Assessing the risk

"We must stack this risk alongside the risk of normal treatment and the dangers of a mismatched bone marrow transplant. Leukaemia is a very serious illness but it can be treated and success rates now exceed 80%."

The conventional treatment for a child with X-SCID is bone marrow transplantation.

This can have excellent results when a fully matched donor is found, but results are less successful with a poorer match.

And only one third of X-SCID patients will have a fully matched donor.

Great Ormond Street is the only hospital in the UK doing this type of gene therapy for children with X-SCID and similar genetic disorders.

"There is no real difference between this and any other form of medicine," Professor Thrasher explains.

"There will be side effects but you have to balance the risks against the benefits.

"Gene therapy has the potential to tackle disorders where there was no treatment at all before, for inherited diseases particularly."

And how is Rhys now?

"We still keep an eye on him as because he's growing, his immune system sometimes needs a helping hand to keep up," replies Prof Thrasher.

"But we're delighted to see he's doing so well eight years after his treatment."