1. Biology Department, Institute of Biochemistry, Swiss Federal Institute of Technology (ETH), ETH-Hönggerberg, 8093 Zürich, Switzerland
2. Institute of Computational Science, ETH, ETH-Zentrum, 8092 Zürich, Switzerland
3. Max Planck Institute of Developmental Biology, D-72076 Tübingen, Germany
4. Friedrich Miescher Laboratory of the Max Planck Society, D-72076 Tübingen, Germany