1. Center for Computational Molecular Biology, Brown University, Providence, Rhode Island, USA.
2. Department of Computer Science, Brown University, Providence, Rhode Island, USA.
3. Division of Applied Mathematics, Brown University, Providence, Rhode Island, USA.
4. Department of Genome Sciences, University of Washington, Seattle, Washington, USA.
5. Paul G. Allen School of Computer Science and Engineering, University of Washington, Seattle, Washington, USA.