[1] Italian Inst Technol, Dept Neurosci & Brain Technol, I-16163 Genoa, Italy
[2] Canc Res UK London Res Inst, Mol Neuropathol Lab, London WC2A 3LY, England
[3] Canc Res UK London Res Inst, Expt Pathol Lab, London WC2A 3LY, England
[4] UCL, Inst Ophthalmol, London EC1V 9EL, England
[5] Canc Res UK London Res Inst, Vasc Biol Lab, London WC2A 3LY, England
[6] UCL, Dept Cell & Dev Biol, London WC1E 6BT, England
[7] UCL, Inst Child Hlth, London WC1N 1EH, England
[8] Max Planck Inst Mol Biomed, D-48149 Munster, Germany