1. Department of Statistical Science, Southern Methodist University, Dallas, TX 75275, USA
2. Quantitative Biomedical Research Center, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
3. Department of Pediatrics, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA and Department of Microbiology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
4. Department of Mathematical Sciences, The University of Texas at Dallas, Richardson, TX 75080, USA