1. D. J. Van Antwerp, Laboratory of Genetics, Salk Institute, La Jolla, CA 92037, and Department of Chemistry and Biochemistry, University of California, San Diego, CA 92093, USA.
2. S. J. Martin and D. R. Green, La Jolla Institute for Allergy and Immunology, San Diego, CA 92121, USA.
3. T. Kafri and I. M. Verma, Laboratory of Genetics, Salk Institute, La Jolla, CA 92037, USA.