1. CEH-Oxford, Mansfield Road, Oxford, OX1 3SR, UK
2. Danish Veterinary Institute, Bülowsvej 27, DK-1790 Copenhagen V, Denmark
3. Center for Biological Sequence Analysis, Institute of BioZentrum-DTU, Technical University of Denmark, Building 208, DK-2800, Lyngby, Denmark
4. Pennsylvania State University, Department of Biology, 208 Mueller Lab., University Park, PA 16802, USA