The never-ceasing pace of scientific accomplishment often surpasses the progress of moral thought, leaving people struggling to make sense, initially at east, of whether heart transplants are ethical or test-tube babies desirable. Over the past three decades scientists have begun to investigate a branch of medicine that offers astonishing promise—the ability to repair the human body and even grow new organs—but which destroys early-stage embryos to do so. In "The Stem Cell Hope" Alice Park, a science writer at Time magazine, chronicles the scientific, political, ethical and personal struggles of those involved in the work. Embryonic stem cells are pluripotent(多功能性的): they have the ability to change into any one of the 200-odd types of cell that compose the human body; but they can do so only at a very early stage. Once the bundle has reached more than about 150 cells, they start to specialize. Research into repairing severed spinal cords or growing new hearts has thus needed a supply of stem cells that come from entities that, given a more favorable environment, could instead grow into a baby. Immediately after the announcement of the birth of Dolly the sheep—the clone of an adult ewe whose mammary(乳腺的)cells Ian Wilmut had tricked into behaving like a developing embryo-American scientists were hauled before the nation's politicians who were uneasy at the implication that people might also be cloned. Concern at the speed of scientific progress had previously stalled publicly funded research into controversial topics, for example, into in vitro fertilization. But it did not stop the work from taking place: instead the IVF industry blossomed in the private sector, funded by couples desperate for a baby and investors who had spotted a profitable new market. That is also what happened with human stem cells. After a prolonged struggle over whether to ban research outright—which pitted Nancy Reagan, whose husband suffered from Alzheimer's disease, against a father who asked George Bush's advisers, "Which one of my children would you kill?"—Mr. Bush blocked the use of government money to fund research on any new human embryonic stem-cell cultures. But research did not halt completely; Geron, a biopharmaceutical(生物制药的)company based in Menlo Park, California, had started "to mop up this orphaned innovation" , as Ms Park puts it, by recruiting researchers whose work brought them into conflict with the funding restrictions. Meanwhile, in South Korea a scientist claimed not only to have cloned human embryos but also to have created patient-specific cultures that could, in theory, be used to patch up brain damage or grow a kidney. Alas, he was wrong. But a Japanese scientist did manage to persuade adult skin cells to act like stem cells. If it proves possible to scale up his techniques, that would remove the source of the controversy over stem-cell research. Three months after he took office, Barack Obama lifted restrictions on federal funding for research on new stem-cell cultures, saying that he thought sound science and moral values were consistent with one another. But progress has been slow: the first human trials in America, involving two people with spinal-cord injuries who have been injected with stem celis developed by Geron, are only just under way. The sick children who first inspired scientists to conduct research into stem cells in order to develop treatments that might help them are now young adults. As Ms Park notes, the fight over stem-cell research is not over, and those who might benefit from stem-cell medicine remain in need.