[1] Univ Coll Dublin, Conway Inst Biomol & Biomed Res, Dublin 4, Ireland
[2] Inst Genet & Biol Mol & Cellulaire, Lab Biol & Genom Struct, Illkirch Graffenstaden, France
[3] European Mol Biol Lab, D-69117 Heidelberg, Germany
[4] European Bioinformat Inst, EMBL Outstn, Cambridge CB10 1SD, England