1. C. E. Finch is in the Neurogerontology Division, Andrus Gerontology Center, and Department of Biological Sciences, University of Southern California, 3715 McClintock Avenue, Los Angeles, CA 90089–0191, USA. R. E. Tanzi is in the Genetics and Aging Unit, Massachusetts General Hospital, Neuroscience Center Building 149, 13th Street, Charlestown, MA 02129, USA.