1. L. H. Hartwell, P. Szankasi, and S. H. Friend are at the Seattle Project, Molecular Pharmacology Department, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA. C. J. Roberts is at Rosetta Inpharmatics, Incorporated, 12040 115th Street NE, Kirkland, WA 98034, USA. A. W. Murray is in the Department of Physiology, University of California at San Francisco, 513 Parnassus Avenue, San Francisco, CA 94142–0444, USA.