The notion that genetic diseases might be cured by correcting or replacing the abnormal gene in the affected tissues is an attractive one, and probably reflects the average educated layman’s concept of gene therapy. In the case of severe combined immunodeficiency due to adenosine deaminase deficiency, such curative gene therapy has in fact been achieved by correction of autologous bone marrow cells ex vivo before returning the marrow stem cells to the patient. There are understandable professional and public fears about gene therapy which might introduce additional “corrected” genes into the germ tissue of individuals and thereby potentially into the permanent human gene pool, and for that reason germ line gene therapy is regarded as unethical throughout the world. The limitation which this places on the treatment of a multisystem disease whose clinical effects begin in fetal life is therefore obvious, and expectations of a cure for cystic fibrosis by gene therapy are intrinsically unrealistic.

An alternative strategy is to look for control, rather than cure, of a disease such as cystic fibrosis by repeated delivery of the normal gene to the target tissue for as long as the patient lives. This is the approach which has been taken by teams on both sides of the Atlantic, and an update on progress is …