In order to better understand the cause of hereditary hearing impairment, we have performed a proteomic analysis of the inner ear proteins using two-dimensional gel electrophoresis. In the process of analysis, we have found very unique properties of the bovine homologue of the human COCH gene product. The COCH gene is responsible for one of the hereditary hearing impairments, DFNA9, and was recently suggested to be a possible genetic factor contributing to Ménière's disease. The Coch protein constitutes 70% of bovine inner ear proteins and is composed of 16 different protein spots, with charge and size heterogeneity. Heterogeneity of this protein suggests that the Coch gene is processed in several ways, at the transcriptional and/or posttranslational level. Much knowledge has accumulated about the hereditary hearing impairment genes; however, little research has been done regarding the protein products of those genes. This is the first report to characterize the Coch protein. Study of the Coch protein might provide more information on the mechanism of hearing and vestibular disorders.