1. Anne McLaren Laboratory, Wellcome Trust and Medical Research Council Cambridge Stem Cell Institute, Forvie Site, University of Cambridge, Robinson Way, Cambridge CB2 0SZ, UK;
2. Department of Medicine, Addenbrookes Hospital, Box 157, Hills Rd, Cambridge, CB2 0QQ, UK;
3. Laboratory of Angiogenesis and Redox Metabolism, Department of Biology, University of Padua, 35131 Padova, Italy