1. Graduate Program in Molecular and Cellular Biology, University of Washington and Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North A3-025, Seattle, Washington 98109, Departments of Genetics and Pathology, Box 357470, University of Washington, Seattle, Washington 98195, and Program in Evolutionary Biology, Department de biochimie, Laval Universite, Quebec City, Quebec G1K 7P4, Canada