1. Beatrix Children's Hospital / University Medical Center of Groningen, University of Groningen; Department of Pediatric Oncology/Hematology; PO Box 30.001 Groningen Netherlands 9700 RB
2. University Medical Center Groningen, University of Groningen; Unit Chronic Airway Diseases, Department of Epidemiology; Triade gebouw, E3.29 P.O.Box 30.001 Groningen Netherlands 9700 RB