David Schaffer spends most of his time making things grow:I n his garden, orchids; in his lab, stem cells. The biomedical engineer is trying to coax stem cells that lie nearly dormant in the brain to multiply at a much quicker rate than they ordinarily do, which could help regenerate damaged nerve tissue in patients with Alzheimer’s or Parkinson’s diseases. Last year Schaffer and his colleagues discovered a protein that causes stem cells to grow, and he showed that the protein’s action could trigger the repair of nerve cells in mice. By determining how the protein works, Schaffer may be able to get neural stem cells in human patients to replace damaged neurons. To carry the protein to stem cells, Schaffer is using inactivated viruses as delivery vans and is now tinkering with their molecular properties to help them find their targets precisely. Schaffer’s background equipped him well for his work: he grew up in a family of doctors, was interested in mathematics and majored in engineering. Prodding stem cells to grow is harder than cultivating orchids, but the potential rewards are richer, too.