1. G. J. Pronk and P. Amiri, Chiron Corporation, Emeryville, CA 94608, USA.
2. K. Ramer, Cardiovascular Research Institute, University of California, San Francisco, CA 94143, USA.
3. L. T. Williams, Chiron Corporation, Emeryville, CA 94608, and Cardiovascular Research Institute, University of California, San Francisco, CA 94143, USA.