The chief medical ambition of those who study stem cells has always been that the cells would be used to repair and regenerate damaged tissue. That's still a long way off, despite rapid progress exemplified by the presentation of the Nobel Prize next week to Shinya Yamanaka of Kyoto University for a key stem-cell breakthrough. But there's another, less well known application of stem cells that is already delivering results: disease modeling. Dr. Yamanaka used a retrovirus to insert four genes into a mouse cell to return it to a "pluripotent" state—capable of turning into almost any kind of cell....