1. R. Freeman is at the National Bureau of Economic Research, Cambridge, MA 02138, USA. R. Freeman, E. Weinstein, and J. Rosenbaum are or were at Harvard University, Cambridge, MA 02138, USA.
2. E. Marincola is at the American Society for Cell Biology, 8120 Woodmont Avenue, Bethesda, MD 20814, USA.
3. F. Solomon is at the Massachusetts Institute of Technology, Cambridge, MA 02139, USA.