Amy J. Wagers and her colleagues at the Stanford University School of Medicine isolated blood-forming stem cells from adult mice in order to test their plasticity. To track the adult stem cells' transformation, the team genetically engineered them to produce a green fluorescing protein that is easily identifiable under a microscope (see image). When mice whose bone marrow had been destroyed by radiation received a single stem cell, green-glowing cells replaced the animals' missing blood and immune cells within a few weeks. A comprehensive search of more than 15 million muscle, brain, liver, kidney, gut and lung cells, however, found only one brain and seven liver cells that appeared green. As a result, the scientists suggest that the capabilities of adult stem cells so far have been overstated. Says Wagers: "It's not to say that nobody should think about adult stem cell plasticity--of course people should look into it--but it's not as robust as it is claimed to be."