1. Department of Physiology, Development and Neuroscience, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK.
2. Division of Biology and Biological Engineering, California Institute of Technology (Caltech), Pasadena, CA 91125, USA.
3. Centre for Stem Cell Biology and Regenerative Medicine, School of Medicine, Tsinghua University, Beijing 100084, China.
4. Centre for Trophoblast Research, University of Cambridge, Cambridge CB2 3EG, UK.