The landmark report by Cavazzana-Calvo et al1 in 2000 of successful gene therapy in two infants with X-linked severe combined immune deficiency (SCID-X1), and a second report this year on the extended wellbeing of these and three further patients2 had convinced the scientific community that gene therapy had at last been accomplished in man. Efforts to achieve gene correction of another type of SCID, caused by adenosine deaminase deficiency, had repeatedly failed in the 1990s, and the success of the group headed by Dr Alain Fischer at the Hôpital Necker in Paris, France, has led investigators around the world to anticipate expanded use of gene therapy for this and the seven other known genetic forms of SCID.

Before 1968, there was no effective therapy for SCID—all children with this syndrome died in the first two years of life. Since then, more than 800 children worldwide have been given bone marrow transplants, the majority in the past 20 years when T-cell depletion techniques have made it possible to use haploidentical (ie, half-matched) parental donors when a matched related donor (usually a sibling) was not available. Although a life-saving procedure for many SCID children, the survival rates following bone marrow transplantation have varied from 46% 3, 4 to 97% 5 at