The phenotypes observed in mice homozygous for Slc25a21tm1a(KOMP)Wtsi were not due to ablation of Slc25a21 function, but instead the presence of the selection cassette affected expression of the neighbouring gene, Pax9. The resulting mutant line confirmed and extended the existing knowledge of Pax9 gene dosage effect on orofacial development and represents a novel model of otitis media that may be due to reduced Pax9 expression. The gene discussed is SLC25A21; the disease is otitis media.