[1] UCL, Inst Struct & Mol Biol, London WC1E 6BT, England
[2] MRC Lab Mol Biol, Cambridge CB2 0QH, England
[3] Univ Cambridge, Dept Biochem, Cambridge CB2 1GA, England
[4] UCL, Dept Comp Sci, London WC1E 6BT, England
[5] European Bioinformat Inst, Hinxton CB10 1SD, Cambs, England
[6] Univ London Imperial Coll Sci Technol & Med, Dept Life Sci, Ctr Bioinformat, London SW7 2AZ, England
[7] Univ Bristol, Dept Comp Sci, Bristol BS8 1UB, Avon, England
[8] Hammersmith Hosp, MRC Clin Sci Ctr, London W12 0NN, England