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The field of stem cell research is changing shape almost as fast as the flexible cells themselves. A series of new techniques has moved the stem cell debate onto new technical, political, and ethical landscape, where it may be possible to find a clear path through the current stalemate.

Before the recent developments, it was more-or-less accurate to talk about two kinds of stem cells: adult and embryonic. Adult stem cells, isolated from a variety of tissues including newborns' umbilical cords, are uncontroversial. Not so embryonic stem cells, which until now have been derived from "surplus" embryos that were created but aren't needed for assisted reproduction.

This two-part typology no longer holds. Depending on how you count, there are now two or three additional stem cell varieties. Each differs from the others in some important details. And in those details lie some significant devils.

­As is well known, many people are uneasy about research that involves destroying human embryos, and some - notably religious conservatives - adamantly oppose it. This is the devil that has dominated the stem cell debate,...