1. Department of Genetics, Development, and Cell Biology, Iowa State University, Ames, Iowa 50011-3260
2. Department of Statistics, Iowa State University, Ames, Iowa 50011-1210
3. Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, California 90095\x{2013}1606
4. Institute of Genomics and Proteomics, University of California, Los Angeles, California 90095-1569
5. Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569
6. Department of Statistics, University of Nebraska, Lincoln, Nebraska 68588-0665