BRITISH doctors treating twin babies dying of a rare disease are preparing to experiment with a high-risk procedure as hopes fade of finding a bone-marrow donor. Blood samples from 1,700 people who registered as donors in Hong Kong last month are still being analysed in the United States, but the chances of finding a match for the half-Chinese boys look increasingly slim. Alex Chan Wen-sang, the weaker of the 11-month-old twins, has been moved to a special sterile unit at London's Great Ormond Street Hospital in preparation for a peripheral stem cell transplant on December 15. The technique was only developed recently and it is the first time it has been tried in Britain on a child with Alex's condition. Doctors will use his father, Alan Chan Wei-hung, as the donor, but because his tissue type is only a partial match the chances of failure are high. Mr Chan said: 'It's quite a hazardous route. I'm extremely worried about whether it will be a success.' Instead of carrying out a straightforward transplant, doctors will give Mr Chan drugs to stimulate his bone marrow to produce stem cells, or baby cells, in the hope they will grow into mature bone-marrow cells when transferred to his son. Alex and his brother, Oscar Chan Wen-ong, have Wiskott-Aldrich syndrome, which means their immune systems are so weak that they are vulnerable to the slightest infection. Doctors will wait to see whether Alex's transplant works before they treat Oscar, who was taken to hospital on Friday with a fever. Mr Chan, 35, a business manager from West London, had pinned his hopes on finding a donor in Hong Kong because his family come from the SAR.