Stem cells, unlike all other cells in the body, can copy themselves indefinitely. So-called adult stem cells are found in many parts of the body, constantly rejuvenating the brain, remodelling arteries so blood can bypass clogs, and growing new skin to heal wounds. However, adult stem cells have more limited power than embryonic stem cells, which can turn into any type of cell in the body. Indeed, scientists are hoping that embryonic stem cells could be turned into neurons to fix damaged brains, cardiac cells to repair damaged hearts, or pancreatic cells to create insulin for people with diabetes. Maybe they could even be used to regenerate whole organs. To date, scientists worldwide have made more than 100 different human embryonic cell lines. Still, the existing lines have serious limitations. Most have been grown on a lattice of mouse embryonic skin cells for support. Consequently, the human embryonic cells are contaminated by mouse cells, and though they’re still useful for research, they cannot at present be used to develop therapies for humans.