Embryonic stem cells, which can be coaxed to turn into any kind of cell type, have been hailed as a 21st-century panacea. But they are fraught with ethical problems because they come from embryos. Last November, two teams of scientists turned ordinary adult skin cells into pluripotent stem cells—capable of becoming any kind of tissue—a feat that could solve the ethical problem forever. Here’s how one group did it.

1. The scientists, led by Shinya Yamanaka of Kyoto University in Japan, identified 24 genes that are active in embryonic stem cells but not in adult cells. They deposited combinations of the 24 genes into the DNA of adult mouse skin cells.

2. They found that just four of the original 24 genes will turn adult cells into stem cells. The scientists aren’t quite sure what the genes do. They think two of the genes code for proteins that encourage further protein synthesis.

3. The scientists repeated their experiment on human adult skin cells using the same four genes as in the mouse model. The human cells also turned into stem cells and then differentiated into brain and heart cells.

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