Ron Weiss likes to give orders. In his lab at Princeton University, the assistant professor of electrical engineering sets the conversation and dictates the action. His charges, however, are not students but cells. Weiss builds synthetic DNA circuits- strings of genes that operate much like the logic circuits of computers- and injects them into E. coli bacteria, where they direct cell behavior. His goal? Create networks of different cells that work together to sense environmental toxins, generate new tissue, or perform other jobs. Collaborating with researchers at Princeton and Caltech, Weiss shares almost $6 million in grants, much of it from the U.S. Defense Advanced Research Projects Agency. In one of his projects, Weiss leads a multiple laboratory effort to program groups of cells to act as biological sentinels. Such systems could detect and pinpoint the locations of anthrax or other biological weapons. In another project, Weiss is devising faster, more reliable ways to direct stem cells to create new tissues to replace those lost to disease or injury- which means that one day a doctor might be able to order your own cells to heal you.