1. University of Chile, Casilla 70055 Santiago 7, Chile
2. Royal Free Hospital School of Medicine (University of London), Rowland Hill Street, London NW3 2PF, U.K.
3. University of Kent, Canterbury, Kent CT2 7NJ, U.K.
4. University of Alberta, Edmonton, Canada T6G 2H7;
5. University of Cambridge, Cambridge CB2 1QW, U.K.