MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK. jkb@mrc-lmb.cam.ac.jp|MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK. jkb@mrc-lmb.cam.ac.jp|MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK. jkb@mrc-lmb.cam.ac.jp|MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK. jkb@mrc-lmb.cam.ac.jp