Gene therapy has been used for the first time to treat an inherited blood disorder in what doctors say is a major step forward.
A man given pioneering treatment to correct a faulty gene has made "remarkable" progress, a US and French team has revealed.
Gene therapy is an experimental technique that manipulates genes in order to treat disease.
It has seen some successes, but also setbacks, including a patient's death.
Beta thalassaemia is an inherited blood disorder that affects the body's ability to create red blood cells.
The first gene therapy trial was in an 18-year-old man with a severe form of the condition, who had been receiving regular blood transfusions since the age of three.
Stem cells from his bone marrow were treated with a gene to correct for the faulty one.
They were then transfused back into his body, where they gradually gave rise to healthy red blood cells.
Three years after the treatment, which took place in 2007, the man remains mildly anaemic, but no longer needs blood transfusions, doctors said.
The team, led by Philippe Leboulch, of Harvard Medical School in Boston, said: "At present, approximately three years post-transplantation, the biological and clinical evolution is remarkable and the patient's quality of life is good."
But, reporting in the science journal Nature, the doctors sounded a note of caution, saying there was a possibility that the patient could be at risk of developing leukaemia in the future due to side effects from the gene therapy.
Gene therapy has been used since the 1990s as a new approach to treating a number of incurable conditions, including inherited disorders, some cancers, and viral infections.
There have been some positive results, but in 1999 an 18-year-old US volunteer, Jesse Gelsinger, died after the treatment.
And some children given gene therapy for the immune disorder "bubble baby" syndrome have developed cancer.
Proof of principle
Prof Adrian Thrasher, of University College London, has carried out gene therapy on children with immune disorders.
He said the latest study was an encouraging proof of principle that gene therapy could have genuine therapeutic effects in other blood disorders.
"The good news is that technology is advancing rapidly, and it shouldn't be too long before diseases such as thalassaemia can be reliably and safely treated in this way," he said.
Dr Derek Persons, of St Jude Children's Research Hospital in Memphis, Tennessee, said the work was "a major step forward for the gene therapy of haemoglobin disorder".
He said further trials were planned at several centres in the US, including his own.
"This is very early days," he added. "The field will advance from people doing different trials."