1 Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA.
2 Mammalian Embryo and Stem Cell Group, Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, UK.
3 The Francis Crick Institute, London, UK.
4 Department of Biochemistry, University of Cambridge, Cambridge, UK.
5 Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA. magdaz@caltech.edu.
6 Mammalian Embryo and Stem Cell Group, Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, UK. magdaz@caltech.edu.