1. The Anne McLaren Laboratory for Regenerative Medicine, Stem Cell Institute, University of Cambridge, Cambridge CB2 0SZ, UK.
2. Department of Surgery, University of Cambridge, Cambridge CB2 0SZ, UK.
3. Sanger Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SA, UK.
4. Developmental Biology Program, Sloan-Kettering Institute, New York, NY 10065, USA.
5. Brigham and Women’s Hospital and Harvard Medical School, 77 Avenue Louis Pasteur, Boston, MA 02115, USA.
6. Centre for Trophoblast Research, Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge CB2 3EG, UK.