1. A. S. Perelson and A. U. Neumann, Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
2. M. Markowitz and D. D. Ho, Aaron Diamond AIDS Research Center, 455 First Avenue, New York, NY 10016, USA.
3. J. M. Leonard, Pharmaceutical Products Division, Abbott Laboratories, Abbott Park, IL 60064, USA.