Scientists have devised a new technique to derive embryonic stem cells in mice, which avoids the destruction of the embryo, a development that could shift the grounds of the longstanding political debate about human stem cell research. Until now the only way of deriving human embryonic stem cells has been to break open the embryo before it implants in the uterus and to take out its inner mass, whose cells form all the tissues in a human body. This has run into opposition from US anti-abortion activists, who say the destruction of any embryo is wrong and who have strenuously opposed federal financing of the research.