Looking better: Shah Rayhman cuddles his son Shah after he underwent gene therapy treatment for a blood disorder. Photograph: Simon Townsley

DOCTORS have achieved the world's first gene therapy cure. Children born with a fatally defective gene for a vital blood component have been effectively treated after the correct DNA was artificially inserted into their cells.

The triumph, hailed by scientists as comparable to the first heart transplant, has enabled the children - one British - to celebrate a Christmas this weekend that their parents were never sure they would see. Last week, tests at the Great Ormond Street hospital for sick children in London confirmed that one-year-old Shah Rayhman, from east London, was making good progress.

In France, where the treatment was carried out, two other babies were found to be doing well and a fourth is still under observation.

Scientists have heralded the achievement as the start of a new era in medicine. In the future, rather than treating symptoms, doctors could rewrite the DNA blueprint for life.

Dr Graham Davies, a consultant paediatric immunologist at Great Ormond Street who is Shah's doctor, said: "This is the first clear demonstration that gene therapy has corrected a condition."

Shah was born with a form of Severe Combined Immuno-Deficiency (SCID), in which a single defective gene prevents the manufacture of T-lymphocytes, the white "killer" cells that circulate in the blood and lymph to fight off invading microbes.

The condition leaves children vulnerable to all kinds of infections and many sufferers have to spend their lives in isolation. Some can be treated with bone marrow transplants if suitable donors can be found, but babies who are untreated usually die before they reach 12 months. Tackling the cause of such conditions by rewriting or replacing the defective genes has long been a dream of doctors.

Gene therapy also offers the possibility of curing diseases such as cancer and heart disease - but despite dozens of trials around the world it has never yet been proven to work. About 3,000 people are taking part in gene therapy trials, with 280 of them in Britain.

The technique used for Shah was a standard one employed in many other trials - suggesting other successes will follow. It involves stripping a harmless retrovirus of some of its genes and replacing them with the correct version of the defective gene that causes a disease. The virus is introduced to the body to infect the target tissues. The advantage of using viruses as a "vector" is that they have been honed by evolution to break into cells and incorporate their genes into the cells' DNA.

The French team, led by Dr Marina Cavazzana-Calvo at the Necker hospital in Paris, took bone marrow out of the four children and then infected it with the virus carrying the corrective gene. After about a week the bone marrow was returned to the babies, one just four weeks old.

After several weeks doctors found the new gene was in place and working. In the two children operated on first, levels of their T-lymphocytes had soared to normal levels.

Adrian Thrasher, Wellcome senior clinical fellow at the Institute of Child Health in London, said he was delighted. "It takes therapy to a new level where we can now successfully use genes as drugs - and it would be reasonable to say it is as important as the first successful heart transplant." he said.

Shah's father, also called Shah, hopes the treatment will work for others. "Shah had a long history of infections and they could not find suitable bone marrow for a transplant," he said. "Then the doctors told us about this new treatment. It seems to have worked and all we can do now is wait."