1. S. F. Yu and S. Yendapalli, Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98104, USA.
2. D. N. Baldwin and M. L. Linial, Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98104, and Department of Microbiology, University of Washington, Seattle, WA 98195, USA.
3. S. R. Gwynn, Interdisciplinary Program in Molecular and Cellular Biology, and Department of Microbiology, University of Washington, Seattle, WA 98195, USA.