E Christensen1,2, P Mjønes3, Ø Grimstad4, O M Rørdam5, O A Foss6. 1. Department of Dermatology, Clinic of Orthopaedy, Rheumatology and Dermatology, St. Olavs Hospital, Trondheim University Hospital, Trondheim, Norway. 2. Department of Cancer Research and Molecular Medicine, Faculty of Medicine, Norwegian University of Science and Technology (NTNU), Trondheim, Norway. 3. Department of Pathology and Medical Genetics, St. Olavs Hospital, Trondheim University Hospital, Trondheim, Norway. 4. Department of Dermatology, University Hospital of North Norway, Tromsø, Norway. 5. Department of Dermatology, Facultad de Medicina, Universidad Complutense, Madrid, Spain. 6. Orthopaedic Research Centre, Clinic of Orthopaedy, Rheumatology and Dermatology, St. Olavs Hospital, Trondheim University Hospital, Trondheim, Norway.