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Conductive hearing loss (CHL) is one of the most common forms of human deafness. Despite this observation, a surprising gap in our understanding of the mechanisms underlying CHL remains, particularly with respect to the molecular mechanisms underlying middle ear development and disease. Treacher Collins syndrome (TCS) is an autosomal dominant disorder of facial development that results from mutations in the gene TCOF1. CHL is a common feature of TCS but the causes of the hearing defect have not been studied. In this study, we have utilized Tcof1 mutant mice to dissect the developmental mechanisms underlying CHL. Our results demonstrate that effective cavitation of the middle ear is intimately linked to growth of the auditory bulla, the neural crest cell-derived structure that encapsulates all middle ear components, and that defects in these processes have a profoundly detrimental effect on hearing. This research provides important insights into a poorly characterized cause of human deafness, and provides the first mouse model for the study of middle ear cavity defects, while also being of direct relevance to a human genetic disorder.