In a technique called altered nuclear transfer, researchers at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, successfully disabled a mouse embryo by altering the gene which controls the development of the placenta. This means that an embryo clone created in this way could never develop into an adult, but it would still yield ESCs. The reasoning behind this technique is that if the embryo is clearly fated never to develop properly, it is ethical to destroy it.

However, even if ESCs can be obtained efficiently this way, it is unlikely to quench the debate. "The success of this procedure in no way precludes the need to pursue all forms of human embryonic stem cell research," says Whitehead researcher and MIT professor Dr Rudolf Jaenisch. "Human embryonic stem cells are extraordinarily complicated. If we are ever to realise their therapeutic potential, we must use all known tools and techniques in order to explore the mechanisms that give these cells such startling characteristics."

The other technique, which is similar to pre-implantation genetic diagnosis, has been proposed by Advanced Cell Technology, a private company in Massachusetts which does cloning research. A team headed by Robert Lanza successfully removed a cell from an 8-cell embryo and used it to cultivate ESCs, without apparent harm to the embryo.

A lawyer for the Bush administration, Todd Henry, called the allegations "outrageous" and described them as "story-telling, exaggerations, misunderstandings, or miscomprehensions of what is going on." However, US district Court Judge Gladys Kessler took the claims seriously and ordered the government to supply a written response with a week.

A growing body of evidence suggests that cancers can be caused by rare mutant stem cells. This might explain why some resist chemotherapy. This targets quick-growing cancer cells, but fails to knock out slow-growing cancer stem cells. These linger on in the body to produce more tumours later. "It's like trying to kill a dandelion," says John Dick, of the University of Toronto. you can kill the leaves, but you have to kill the taproot to kill the plant." Dr Dick feels that stem cells may be "the apex of all" malignant growth.

Until recently many researchers were sceptical about the stem cell theory of cancer. But over the last two years, the evidence has become more persuasive. Significant experiments have shown that injecting tens of thousands tumour cells into a mouse was not enough to cause cancers. But if as few as a hundred cancer stem cells were injected, tumours were triggered.

As a seminar organised by the Center for American Progress, a left- leaning Washington thinktank with close links to the Democratic Party, several "progressive" bioethicists described their initiative in terms which suggest that US bioethics has fissured into nearly irreconcilable camps operating with completely different philosophical frameworks.

According to the speakers at the seminar, progressives believe that bioethics should concern itself with social justice issues, and that human dignity is best promoted by providing the necessities of life, like healthcare, food and education. Conservatives, on the other hand, want "government enforcement of majoritarian prejudices that are based on emotion and instinct and repugnance". To use the words of R. Alta Charo, of the University of Wisconsin, Madison, its recent interest is "foetus fetishism".