[1] UNITED MED & DENT SCH,GUYS HOSP,DEPT EXPTL PATHOL,LONDON SE1 9RT,ENGLAND
[2] UNIV LONDON,ROYAL HOLLOWAY & BEDFORD NEW COLL,SCH BIOL SCI,DIV BIOCHEM,EGHAM TW20 0EX,SURREY,ENGLAND
[3] UNIV IOWA,HOWARD HUGHES MED INST,IOWA CITY,IA 52242
[4] UNIV IOWA,DEPT PHYSIOL & BIOPHYS,IOWA CITY,IA 52242