Retroviruses could, by chance, land near human genes linked to cancer and turn them on. So scientists are looking for alternative carriers. Even if patients are prepared to take these risks, treatment is only temporary. The cells transformed by genetic engineering live out their normal lifespans - in some cases just a few weeks. So treatment lasts only as long as they do, and cells have to be reintroduced every few months.
Researchers continually look for ways to extend treatment.
The gene therapy experiments approved so far involve adding new genes to the cells where they are needed - in the bone marrow, for instance. So the genetic change is not passed on to the patient's offspring. Every cell carries the same package of DNA, including sex cells - the sperm and egg. At the moment this is generally regarded as technically impossible, and the ethics have not been properly addressed. Any work involving human subjects will be covered by the existing Medical Research Council guidelines.