Within 6 months, the two or three dozen cells taken from a single embryo can generate millions of embryonic stem cells. That gives scientists enough cells to complete and repeat experiments and allows them to ask questions about disease that would be impossible with other kinds of cells.

Credit: Carol Marchetto and Fred H. Gage / The Salk Institute

Embryonic stem cells are easy to isolate and purify, at least in comparison with most tissue-specific cells, which exist in vanishingly small numbers deep within tissues. Although human embryonic stem cells can multiply in the lab for years without differentiating into more specialized cells, these cells are believed capable of forming every kind of cell in the human body, given the right conditions. (Mouse embryonic stem cells have been demonstrated to form every kind of cell in a mouse.)