1 Helmholtz Centre for Infection Research and Hannover Medical School, 38124 Braunschweig, Germany; The Laboratory for Molecular Infection Medicine Sweden, Umeå University, Umeå, Sweden. Electronic address: emmanuelle.charpentier@helmholtz-hzi.de.
2 The Rockefeller University, 1230 York Avenue, New York, NY 10065, USA. Electronic address: Luciano.Marraffini@rockefeller.edu.