1. Department of Genetics and Biotechnology, Faculty of Agricultural Sciences, University of Aarhus, DK-8830 Tjele, 2Department of Basic Animal and Veterinary Sciences, Faculty of Life Sciences, University of Copenhagen, DK-1870 Frederiksberg C and 3Center for Biological Sequence Analysis, BioCentrum-DTU, Building 208, DK-2800 Lyngby, Denmark