In a pair of landmark studies, two groups of scientists announced that they have successfully reprogrammed human skin cells to behave like embryonic stem cells, cells that have the potential to mature into any type of tissue in the body. Stem cells have long been the source of ethical, legal, and political debate; and while their cultivation could lead to cures for numerous diseases, harvesting of these vital cells often requires the destruction of fertilized(受精した)embryos(胚). The new embryo-free technique involves the use of a retrovirus to introduce new genes into the skin cells and sidesteps(避ける)the long-running ethics debate.