1. Department of Medical Genetics, Medical University-Sofia, 2 Zdrave St., Sofia 1431, Bulgaria
2. Institute for Cell Biology, Department of Immunology, University of Tübingen, Auf der Morgenstelle 15, D-72076 Tübingen, Germany
3. Max Planck Institute for Developmental Biology, Spemannstraße 35, D-72076 Tübingen, Germany
4. Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 9 G. Bonchev St., Sofia 1113, Bulgaria