1. Institute of Human Genetics, GSF-National Research Center for Environment and Health, Ingolstaedter Landstr. 1, D-85764 Neuherberg, Germany, Institute of Animal Physiology, LMU Munich, Veterinärstr. 13, D-80539 Munich, Germany, Department of Equine Surgery and Medicine, LMU Munich, Veterinärstr. 13, D-80539 Munich, Germany, and Institute of Human Genetics, Technical University of Munich, Trogerstr. 32, D-81675 Munich, Germany