Institute for Theoretical Computer Science (ITCS), Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing 100084, P. R. China, Department of Computer Science, Duke University, Durham, NC 27708, USA and Department of Biochemistry, Duke University Medical Center, Durham, NC 27708, USAInstitute for Theoretical Computer Science (ITCS), Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing 100084, P. R. China, Department of Computer Science, Duke University, Durham, NC 27708, USA and Department of Biochemistry, Duke University Medical Center, Durham, NC 27708, USA.