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* Organs and tissue have already been transplanted from animals into humans but organs genetically engineered to prevent rejection by the human immune system have not. In 1994 an Aids patient had a bone-marrow graft from a baboon that failed to take. Much attention focuses on the possibility of transplanting genetically engineered organs such as hearts, kidneys andlivers from pigs.

* How much more research before xenotransplants involving humans become commonplace? Richard Nicholson, editor of the Bulletin of Medical Ethics, argues that at least 50 per cent of pig-to-monkey xenografts, in a cohort of 10 monkeys, should be alive and well after two years before any ethics committee approve human trials. Imutran, one of the three leading companies worldwidepursuing research in this field, has said that trialtransplants of genetically engineered animal organs into humans might be no more than a year away.

* But last year Imutran, based in Cambridge, announced the results of removing kidneys from seven Cynomolgus monkeys and replacing them with genetically engineered pig kidneys. One monkey died at six days of ureteric obstruction, two at eight days of acute vascular rejection and the remaining four were killed at between 13 and 35 days owing to severe anaemia.

* Much money is riding on xenotransplants. Investment bank Salomon Brothers has suggested that Sandoz, which owns Imutran, could increase its annual income by $7-8 billion by 2010 with sales of organs and a drug that prevents organ rejection.

* The ethical dilemmas of xenotransplantation were considered by the Nuffield Council on Bioethics and the US Institute of Medicine. Both were concerned about the possible spread of infectious diseases by xenograft. Earlier this month a Department of Health committee, chaired by Ian Kennedy of King's College, London, recommended setting up a national regulatory body to approve experiments and monitor progress. The Kennedy report is being followed by three months consultation before final decisions are taken. Delays could benefit Imutran's US rivals, Nextran and Alexion.