Stem cell therapies without tears

Expect to hear a familiar refrain over the coming days: "There's no need for embryonic stem cells." It is the likely response of those who oppose any interference with human embryos to the exciting news that adult mouse cells can be reprogrammed to an embryonic state by boosting the activity of just four critical genes (see "From adult to embryo").

There are good reasons to question the campaigners' claims. The trick has yet to be achieved in human cells. Even if that can be done, it will still be necessary to show that the reprogrammed cells have the same ability as embryonic cells to develop into different tissues. But let's assume that all these issues can one day be resolved. Would the need for cells isolated from embryos disappear?

The eventual goal of the latest work is to derive stem cell lines genetically matched to patients, from which to grow replacement cells and tissues. Transferring the nucleus of an adult cell into an egg can send it back to an embryonic state, but how such cloning works remains mysterious. Reprogramming...