Everything about Shinya Yamanaka's discovery was right—except for the timing. The 44-year-old Kyoto University stem-cell researcher had found a way to genetically reprogram an ordinary mouse skin cell to revert to the virtual equivalent of its embryonic state, in which it has the potential to grow into any kind of tissue. The finding was a promising first step toward the creation of stem-cell lines for near-miraculous medical treatments—and because Yamanaka did not use human embryos, his technique offered researchers everywhere a way to sidestep the ethical controversies that have dogged the field since its birth. But it was March 2006,...