1. The Jackson Laboratory, Bar Harbor, Maine 04609
2. Laboratory of Genetics, University of Wisconsin, Madison, Wisconsin 53706
3. Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford OX3 7BN, United Kingdom
4. Breeding Biology and Molecular Genetics, Institute of Animal Sciences, Humboldt-Universität zu Berlin, 10115 Berlin, Germany
5. J. L. Pettis Memorial VA Medical Center, Loma Linda, California 92357 and
6. Department of Biostatistics and Medical Informatics, University of Wisconsin, Madison, Wisconsin 53706