The Roslin Institute and Royal (Dick) School of Veterinary Studies, Division of Developmental Biology, University of Edinburgh, Easter Bush Campus, Midlothian, EH25 8GR, UK and Department of Haematology, Wellcome Trust and MRC Cambridge Stem Cell Institute, Cambridge Institute for Medical Research, Cambridge University, Cambridge, UK.|The Roslin Institute and Royal (Dick) School of Veterinary Studies, Division of Developmental Biology, University of Edinburgh, Easter Bush Campus, Midlothian, EH25 8GR, UK and Department of Haematology, Wellcome Trust and MRC Cambridge Stem Cell Institute, Cambridge Institute for Medical Research, Cambridge University, Cambridge, UK.|The Roslin Institute and Royal (Dick) School of Veterinary Studies, Division of Developmental Biology, University of Edinburgh, Easter Bush Campus, Midlothian, EH25 8GR, UK and Department of Haematology, Wellcome Trust and MRC Cambridge Stem Cell Institute, Cambridge Institute for Medical Research, Cambridge University, Cambridge, UK.|The Roslin Institute and Royal (Dick) School of Veterinary Studies, Division of Developmental Biology, University of Edinburgh, Easter Bush Campus, Midlothian, EH25 8GR, UK and Department of Haematology, Wellcome Trust and MRC Cambridge Stem Cell Institute, Cambridge Institute for Medical Research, Cambridge University, Cambridge, UK.|The Roslin Institute and Royal (Dick) School of Veterinary Studies, Division of Developmental Biology, University of Edinburgh, Easter Bush Campus, Midlothian, EH25 8GR, UK and Department of Haematology, Wellcome Trust and MRC Cambridge Stem Cell Institute, Cambridge Institute for Medical Research, Cambridge University, Cambridge, UK.