Department of Pediatrics, University of Groningen, University Medical Center Groningen, Groningen, the Netherlands Correspondence to Dr Uwe J.F. Tietge, Department of Pediatrics, University Medical Center Groningen, Hanzeplein 1, 9713 GZ Groningen, the Netherlands. Tel: +31 50 3636762; fax: +31 50 3611746; e-mail: [email protected]