Four UK children born with a condition robbing them of their natural defences against infection have had successful gene therapy treatment, doctors say.
Rhys Evans was the first child to receive the treatment in the UK
Gene therapy adds to the body, via a leukaemia virus, a fully functioning version of the mutated gene that causes severe combined immunodeficiency.
There have been concerns the therapy can cause cancer in some patients.
But all four boys were doing well and had had no serious side-effects, London doctors told the Lancet journal.
Severe combined immunodeficiency (SCID) is passed on from parents to their offspring through the X chromosome.
It is a rare condition, affecting about one in every 50,000-100,000 births in the UK, and only boys.
Without treatment, children with SCID are forced to live in protective "bubbles" to shield them from potentially fatal infections.
The majority of patients can be treated with a bone marrow or stem cell transplant from a donor.
But it can be difficult to find a suitable donor who is a close enough match and the child may be too sick to have this treatment, which involves chemotherapy.
In recent years, researchers have used pioneering gene therapy to cure some children with SCID, including the UK's first child to have this treatment, Rhys Evans, who is fit and well and has celebrated his fourth birthday.
However, two cases of leukaemia among 11 children treated in this way at the Necker Hospital, in Paris, raised alarm among researchers.
In the latest study, Dr Adrian Thrasher and colleagues at the Institute of Child Health, London, report they have successfully treated four boys with SCID identically, with no serious side-effects.
Two of the children have even been able to stop all of their other medication.
None have shown any signs of cancer, although it is still early days and the risk has yet to be clearly defined, the researchers said in the Lancet.
Researcher Dr Bobby Gaspar said: "The major advantages of this therapy are that you are using the child's own cells so you are avoiding the fight between the donor cells and the patient's cells.
"Secondly, we do not have to use chemotherapy. It's proving to be a very effective treatment.
"We hope this paves the way for developing gene therapies for other genetic disorders."
He said this might include treatments for other forms of inherited immunodeficiencies or genetic disorders of the blood, such as sickle cell anaemia.
Dr Marina Cavazzana-Calvo, one of the researchers at the Necker Hospital, said: "This paper is very important. It shows that this approach is highly reproducible.
"The safety issue is always a concern, but we hope that it will be limited to only the two boys who have developed acute lymphoblastic leukaemia so far.
"Unfortunately, we cannot know whether any of the other boys will develop a similar side-effect."
She said none of the other boys were showing any signs of cancer to date.
She said they were also working to find a safer way to deliver the gene therapy and were limiting the treatment to children who had no other hope of a cure.
Dr Peter Fraser, a UK researcher at the Babraham Institute in Cambridge who has also been investigating the safety of gene therapy, said: "There is little doubt that this is a highly successful therapy for immunodeficiencies, however it may be too early to fully assess the potential for adverse affects in these patients.
"Although the prognosis looks good, further studies and careful monitoring of these patients over time are needed before gene therapy can become standard."
Professor Andrew Cant, consultant in paediatric immunology and infectious disease at Newcastle General Hospital, said: "It is a new form of treatment that has promise and, for some children, it will be the best form of treatment that can be given to them."
But he said in about 90% of SCID cases, a cure other than gene therapy could be used.
"For each therapy, you have to evaluate the risks and benefits for the individual child.
"Currently, bone marrow transplant techniques have improved, so if there is a well matched donor that is safer than using gene therapy.
"If you do not have that, particularly in a child who is ill with infection, gene therapy is probably safer than a mismatched transplant.
"It's important that this work is done and goes on," he said.