1. Departments of Muscle Biology and Growth and Genetics and Biometry, Research Institute for Biology of Farm Animals, Wilhelm-Stahl-Allee 2, D-18196 Dummerstorf, Germany, Institute of Grassland and Environmental Research, Aberystwyth SY23 3EB, United Kingdom, and Institute of Biochemistry and Food Chemistry, Department of Food Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg