Mammalian embryonic stem (ES) cells have the special property of being able to differentiate into virtually every cell type. Because ES cells can be genetically manipulated in vitro and can be transplanted into embryos and adults, they are a powerful tool in biological experiments and hold promise for future medical therapies.

The ability to differentiate into all cell types, a property known as pluripotency, arises from the fact that ES cells are isolated from in vitro outgrowths of early stage embryos (in the mouse, at three and one-half days, at the blastocyst stage). These outgrowths are cultured in specialized conditions—often in the presence of support cells, called feeder cells, which do not proliferate, and specific growth factors. The ES cells proliferate rapidly in culture, and clonal (identical) populations can readily be initiated from single cells.

Until 1998 the only mammalian embryonic stem cells isolated...