MRC Laboratory of Molecular Biology, Cambridge, United Kingdom. guilhem@mrc-lmb.cam.ac.uk|MRC Laboratory of Molecular Biology, Cambridge, United Kingdom. guilhem@mrc-lmb.cam.ac.uk|MRC Laboratory of Molecular Biology, Cambridge, United Kingdom. guilhem@mrc-lmb.cam.ac.uk|MRC Laboratory of Molecular Biology, Cambridge, United Kingdom. guilhem@mrc-lmb.cam.ac.uk