Adult SLC12A2 knockout mice do not show the cystic fibrosis–like lung phenotype seen in our proband, presumably due to compensatory mechanisms not found in tracheal epithelia from adult rabbits and humans.20,21 The impaired hearing and the gastrointestinal problems of our proband are likewise described in SLC12A2 knockout mice.22, –, 24. This evidence concerns the gene SLC12A2 and cystic fibrosis.