1. Division of Molecular Embryology, Deutsches Krebsforschungszentrum, Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany.
2. Advanced Light Microscopy Facility, European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, 69117 Heidelberg, Germany.
3. MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.