Bone marrow cell were taken from the SCID-X patient, selected for immature T cell and stimulated with a mixture of cytokines. This was necessary because retroviruses can only infect growing cells. After infection with the modified virus. the cell were injected back into the patient where they continued to grow and produce the normal … receptor. The effect was spectacular in a clinical trial: the SCID-X patients started develop immune responses and where able to live normal lives. However, two out of the 10 developed leukaemia (another five treated in England did not.) This caused the trial to be stopped while investigations were carried out to understand the problems. In both cases of leukaemia the provirus had inserted (probably in very few cell) close to a gene