Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom. elevy@mrc-lmb.cam.ac.uk|Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom. elevy@mrc-lmb.cam.ac.uk|Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom. elevy@mrc-lmb.cam.ac.uk|Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom. elevy@mrc-lmb.cam.ac.uk