1 Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD 21205, USA; Institute for Cell Engineering, Johns Hopkins University, Baltimore, MD 21205, USA. Electronic address: patrick.cahan@jhmi.edu.
2 Department of Biosystems Science and Engineering, ETH Zürich, Basel, Switzerland; Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany; Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany. Electronic address: barbara.treutlein@bsse.ethz.ch.