1. Umeå Center for Molecular Pathogenesis, Building 6L, Umeå University Umeå S‐901 87 Sweden
2. Department of Pathology, Building 6M, Umeå University Umeå S‐901 87 Sweden
3. Molecular and Cell Biology Laboratory, The Salk Institute 10010 N Torrey Pines Road La Jolla California 92037‐1099 USA