[1] ROYAL POSTGRAD MED SCH, DEPT CLIN PHARMACOL, LONDON W12 0HS, ENGLAND
[2] UNIV LONDON IMPERIAL COLL SCI & TECHNOL, DEPT BIOCHEM, LONDON SW7 2AZ, ENGLAND
[3] UNIV LONDON IMPERIAL COLL SCI & TECHNOL, LONDON SW7 2AY, ENGLAND
[4] LAB MOLEC BIOL, CAMBRIDGE CB2 2QH, ENGLAND