1 Beatrix Children's Hospital, Department of Pediatric Pulmonology and Pediatric Allergology, University of Groningen, University Medical Center Groningen, Groningen, 9700 RB Groningen, Netherlands; Groningen Research Institute for Asthma and COPD, University of Groningen, University Medical Center Groningen, Groningen, 9700 RB Groningen, Netherlands.
2 Beatrix Children's Hospital, Department of Pediatric Pulmonology and Pediatric Allergology, University of Groningen, University Medical Center Groningen, Groningen, 9700 RB Groningen, Netherlands; Groningen Research Institute for Asthma and COPD, University of Groningen, University Medical Center Groningen, Groningen, 9700 RB Groningen, Netherlands. Electronic address: g.h.koppelman@umcg.nl.