1 Danish Cancer Society Research Center, Copenhagen, Denmark & Department of Clinical Medicine, Faculty of Health, Aarhus University and Aarhus University Hospital, DK-8200 Aarhus, Denmark. Electronic address: jeanette@cancer.dk.
2 Princess Maxima Center for Pediatric Oncology, Utrecht & Emma Children's Hospital, Amsterdam UMC, University of Amsterdam, Department Pediatrics, Amsterdam, The Netherlands.