1 Nuffield Department of Medicine, University of Oxford, Old Road Campus, Headington, Oxford OX3 7BN, UK. Electronic address: sarah.briggs@ndm.ox.ac.uk.
2 The Christie NHS Foundation Trust, Manchester, UK.
3 Beatson West of Scotland Cancer Centre, Glasgow, UK.
4 Department of Oncology, University of Cambridge, Hutchison/MRC Research Centre, Cambridge Biomedical Campus, Cambridge, UK.