1. Department of Biosciences, Rice University, Houston, TX 77005, USA.
2. Department of Mathematical Sciences, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.
3. Mathematical Biosciences Institute, The Ohio State University, Columbus, OH 43210, USA.
4. Department of Mathematics, University of Houston, Houston, TX 77204, USA.
5. Department of Biology and Biochemistry, University of Houston, Houston, TX 77204, USA.
6. Institute of Biosciences and Bioengineering, Rice University, Houston, TX 77005, USA.