1 MRC Laboratory for Molecular Cell Biology, University College London, Gower Street, London WC1E 6BT, UK; Department of Genetics, Evolution and Environment, University College London, Gower Street, London WC1E 6BT, UK; Molecular Medicine Unit, UCL Institute of Child Health, University College London, 30 Guilford Street, London WC1N 1EH, UK. Electronic address: s.mole@ucl.ac.uk.