1. a Medical Research Council, Laboratory of Molecular Biology , Hills Road, Cambridge , CB2 2QH , UK
2. b Nycomed Amersham plc, Amersham Laboratories , White Lion Road Amersham, Buckinghamshire , HP7 9LL , UK
3. c Amersham Pharmacia Biotech , 800 Centennial Ave., P O Box 1327, Piscataway , NJ , 08855