The technique developed by US researchers is similar to pre-implantation genetic diagnosis, where a single cell is detached from a cluster for analysis to see whether it carries a genetic mutation. Such a cell can be used to develop a line in the laboratory, while the other cells, instead of being destroyed, can be reimplanted in the womb. The resulting stem cells, which have the ability to develop into any kind of tissue, can be used only by the human being that develops from the other cells. To be of therapeutic use, they would would have to be frozen and stored in case of need — a process that would appear impractical.

At least, however, this answers the objections of those who maintain that every fertilised embryo is a potential life and must not therefore be destroyed as it has moral value. Fundamentalists, however, might still argue that even the single detached cell can develop into a human embryo.

More controversial is the more important research into the use of stem cells that can be implanted into another human being to repair tissue or rectify genetic abnormalities. This research arouses stronger passions, as it is based on cloning — and has the potential to create a viable human being. Scientists believe that the therapeutic potential of such research is vast, and could be used to treat Alz- heimer’s, heart disease, strokes, arthritis, diabetes and burns, as well as giving those with spinal injuries hope that one day they may walk again.

In Britain, such research is widely supported. There has been lengthy public discussion, guided by the Human Embryo and Fertilisation Authority, which has kept political polemic at arm’s length. There is a consensus that an embryo at a very early stage is morally neutral, and little objection therefore to its use in important medical research — pro- vided ethical guidelines on the harvesting, techniques and purpose of such research are strictly followed.

In America there is no such consensus. President Bush was persuaded to withdraw federal funding in 2001 by those who believed that any use of fertilised embryos was unethical. Leading US laboratories and universities, however, were horrified, seeing the potential for vital research slip away to Europe. The issue even became part of the last election campaign. This latest move may now give Mr Bush the flexibility he needs to change his mind. That would be a breakthrough as least as important as the scientific advance announced yesterday.

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