1. Institute of Biochemistry and Molecular Cell Biology, Biocenter, University of Vienna, A-1030 Vienna, Austria;
2. Cell Biology Graduate Group, University of Pennsylvania, Philadelphia, Pennsylvania 19104;
3. Cell Biology Group, Worcester Foundation for Biomedical Research, Shrewsbury, Massachusetts 01545;
4. Department of Animal Biology, University of Pennsylvania School of Veterinary Medicine, Philadelphia, Pennsylvania 19104; and
5. Institute of Zoology, University of Rostock, D-18051 Rostock, Germany