Center for Computational Molecular Biology and Department of Computer Science, Brown University, Providence, RI 02912, USA.|Center for Computational Molecular Biology and Department of Computer Science, Brown University, Providence, RI 02912, USA.|Center for Computational Molecular Biology and Department of Computer Science, Brown University, Providence, RI 02912, USA.|Center for Computational Molecular Biology and Department of Computer Science, Brown University, Providence, RI 02912, USA.