Cells generated from bone marrow transplants can migrate from the circulation into the brain and differentiate into neurons, according to a report in Science (290, 1779–1782, 2000). Spurred on by evidence that bone marrow cells can transform into glial cells, Éva Mezey and her coworkers looked to see if blood cells could also metamorphose into neurons. The researchers injected bone marrow from adult male mice into one-day-old female pups of a strain of mouse genetically devoid of bone marrow. Cells derived from the donated marrow could then be detected using fluorescent in situ hybridization of the Y chromosome. After one to four months, donor-derived cells not only took up residence in the bone marrow, spleen, and liver of the mice, but also in the CNS. Furthermore, many of the cells in the brain expressed two neuron-specific markers, suggesting that differentiation had taken place. The next task, says Mezey, is to determine if the process occurs naturally in humans, and what role it might play physiologically. Some answers could come from studies of postmortem brain tissue from women who received bone marrow transplants from males. Nevertheless, Mezey's research raises the exciting possibility that an individual's own bone marrow could be used for CNS transplantation procedures, circumventing the current ethical and practical obstacles associated with embryonic stem cells.