Ya-Chieh Hsu1 and Elaine Fuchs2 1Department of Stem Cell and Regenerative Biology, Harvard University and Harvard Stem Cell Institute, Cambridge, Massachusetts 02138, USA 2Robin Chemers Neustein Laboratory of Mammalian Cell Biology and Development, Howard Hughes Medical Institute, The Rockefeller University, New York, New York 10065, USA Correspondence: yachieh_hsu{at}harvard.edu; fuchslb{at}rockefeller.edu