Department of Genetics, Systems Biology Centre, University of Cambridge, Cambridge, United Kingdom.|Department of Haematology, Wellcome Trust and MRC Cambridge Stem Cell Institute and Cambridge Institute for Medical Research, University of Cambridge, Cambridge, United Kingdom.|Department of Haematology, Wellcome Trust and MRC Cambridge Stem Cell Institute and Cambridge Institute for Medical Research, University of Cambridge, Cambridge, United Kingdom.|Department of Genetics, Systems Biology Centre, University of Cambridge, Cambridge, United Kingdom.