1. Structural Biology and Biocomputing Programme, Macromolecular Crystallography Group c/Melchor Fdez. Almagro 3 28029 Madrid Spain
2. Molecular Oncology Programme, DNA Replication Group, Spanish National Cancer Research Center (CNIO) c/Melchor Fdez. Almagro 3 28029 Madrid Spain
3. Department of Chemistry, University of Cambridge Lensfield Road Cambridge CB2 1EW UK