1. National Veterinary Institute, Ullevaalsveien 68, P.O. Box 8156 Dep., 0033 Oslo, Norway; Department for Plant Genetics and Breeding, Centre for Agricultural Research, Caritasstraat 21, B-9090 Melle, Belgium; and Food Products and Consumer Goods Unit, Institute for Health and Consumer Protection, European Commission, Joint Research Centre, I-21020 Ispra (VA), Italy