Carly Todd, aged eight months, from Lennoxtown, near Glasgow, has the same immune system deficiency which killed her 14-month-old brother four days before she was born.
Children with this rare condition do not make an enzyme called Adenosine Deaminase, or ADA, which is necessary for the immune system to develop properly. Without ADA, Carly is vulnerable to any infection which a baby with normal cells would be able to fight.
Doctors at Great Ormond Street hospital, London, working with scientists at the University of Leiden in the Netherlands, plan to reprogramme Carly's cells so that she can protect herself from infection.
A copy of the ADA gene will be inserted into the cells of Carly's bone marrow which are responsible for making the cells of the immune system.
Dr Gareth Morgan, senior lecturer in immunology, described Carly as a delightful and responsive baby. He said he was 'cautiously optimistic' of success.
One Italian and two American children with ADA have been treated with a different type of gene therapy, aimed mostly at mature cells, but its effect is short lived as the cells have a limited life and the treatment has to be repeated.
The British and Dutch technique aims for a lifetime cure. This is because the bone marrow cells are stem or progenitor cells.
Today, Dr Morgan will take about a cupful of Carly's bone marrow from her pelvis. It will be sent to the laboratory of Dr Dinko Valerio, in Leiden, where the stem cells will be harnessed with the ADA gene copies. The 'incubation' of the newly programmed cells takes about a day and a half. The gene brew will then be returned to Great Ormond Street.
Dr Morgan said that on about day four Carly will be given back the treated cells, probably via a drip. After two weeks a series of blood tests will begin to look for tell-tale chemical changes in the cells from her bone marrow.
He said that after two or three months they would be reasonably confident that the gene transplant had worked. 'This is the first time and we must be realistic. It could be that the cells die off for some reason inside her body. But we are hopeful this will not happen. This sort of gene therapy does not have side effects and should be very advantageous for Carly.'
Special permission for this attempt to manipulate the genes of a human was given by the Clothier committee, the Government's advisory body on the ethics of gene therapy.