The derivation of embryonic stem (ES) cells from pre-implantation mouse embryos was first reported in 19811,2, although it was not until some 17 years later that human ES cells were successfully isolated3. Few could have predicted that nearly a quarter of a century later, murine ES cells, and their human counterparts, would be under such intense investigation and have sparked so much political, media and public debate. This article gives an overview of our current understanding of how the behaviour of mouse and human ES cells is regulated, focusing in particular on the extrinsic and intrinsic signals involved.

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