Mutations in the CDKN1C gene are associated with the childhood developmental disorder Beckwith-Wiedemann syndrome (BWS), whose main symptom is overgrowth of body and organ size. Cdkn1c-knockout mice do not show overgrowth at birth, casting doubt on the validity of mice as an appropriate BWS model. Now, Tunster et al. show that CDKN1C-deficient mouse foetuses do overgrow, but the effect is lost shortly before birth if the knockout foetuses are part of a large litter. This might have important implications for BWS underdiagnosis in non-singleton pregnancies in humans. Page 814

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