A companion journal to Methods in Enzymology focusing on rapidly developing techniques EDITOR Kenneth W. Adolph University of Minnesota Medical School Minneapolis, Minnesota EDITORIAL BOARD Michael D. P. Boyle Kathryn Calame David T. Denhardt Jack E. Dixon Russell F. Doolittle Eva Engvall Donald R. Helinski Tomas Ho¨kfelt Tasuku Honjo Laurence H. Kedes Kenneth K. Kidd M. Daniel Lane Laurence A. Lasky William J. Lennarz David M. J. Lilley Lawrence A. Loeb William F. Loomis Ian R. Mackay Joachim Messing Donald W. Pfaff Eric J. Stanbridge Lap-Chee Tsui M. H. V. Van Regenmortel Sherman M. Weissman Methods in Enzymology Editors-in-Chief: John N. Abelson and Melvin I. Simon