RG Development & Disease, Max Planck Institute for Molecular Genetics, 14195 Berlin, Germany; Institute for Medical and Human Genetics, Charité Universitätsmedizin Berlin, 13353 Berlin, Germany; Charité - Universitätsmedizin Berlin, BCRT - Berlin Institute of Health Center for Regenerative Therapies, 10178 Berlin, Germany. Electronic address: ibrahim@molgen.mpg.de.