Rhys can now play outside
Scientists at Great Ormond Street Hospital in London successfully treated 18-month-old Rhys Evans, who had a condition preventing him from developing an immune system.
The problem, called severe combined immunodeficiency (SCID) is caused by a single mutated gene, and meant that he had to live in sterile conditions or risk picking up a life-threatening infection.
Only one in between 50,000 and 100,000 births in the UK is affected by the disorder.
However, following the ground-breaking treatment, which adds in a correctly-functioning version of the faulty gene, it is hoped that he will be able to lead a normal life.
And doctors are hopeful that children with other similar genetic disorders could benefit in a similar way.
Gene therapy for SCID - often dubbed the "baby in a bubble" syndrome - has only been successful in two other cases, treated at a Paris Hospital in 2000.
Gene therapy hurdles
In general, attempts to use gene therapy in a wide variety of medical fields have had only limited success so far.
This is because it has proved difficult to get the therapy to the cells which need it.
However, the Great Ormond Street team, led by Dr Adrian Thrasher, is now moving on to tackle other similar disorders caused by a single gene defect.
"After his gene therapy, he was running around at home - he's a normal little boy now."
Rhys' mother Marie said that she had been restricted to fortnightly visits because Rhys had to stay at Great Ormond Street while she returned home.
She said: "It is difficult to look at what we have been through - we see him now, playing with other children, and it's just amazing.
"Since they cracked the genetic code and found the gene, they can work miracles."
No donor
The only existing treatment for SCIDs is bone marrow transplantation, but in the case of Rhys, a matching donor could not be found.
Instead, Dr Thrasher's team took bone marrow from the boy, then used a virus to carry a new version of the gene into immune cells from the marrow.
This was then reimplanted into the patient, where it gradually began to generate further cells to pass into the bloodstream and protect him from infection.
Now he has a normal count of immune cells for a child of his age, and doctors are hopeful this will continue.
This particular type of SCID, called "X-linked" SCID, affects only boys.
The scientists now want to try to correct an illness called chronic granulamatous disorder (CGD), which again invovles a single defective gene.
The success is the first for the newly-launched gene therapy laboratory at Great Ormond Street, which was partly funded by the trust's "Jeans for Genes" appeal.