1. Department of Biology (A.S.I.-P., Y.H., H.B., P.N.B.), Institute for Genome Sciences and Policy Center for Systems Biology (A.S.I.-P., Y.H., H.B., J.H., P.N.B.), and Department of Mathematics (Y.M., J.H.), Duke University, Durham, North Carolina, 27708; and School of Biology (O.S., Y.M., J.S.W.) and School of Physics (J.S.W.), Georgia Institute of Technology, Atlanta, Georgia, 30332