TMC1 and hearing loss disorder: In normal hearing mice, both IHCs and OHCs express WT Tmc1, thus, Tmc1 gene replacement in both cell types is required to overcome the consequences of recessive loss-of-function mutations in Tmc1. Thirty-five TMC1 mutations have been identified in humans29, including several with moderate-to-severe hearing loss phenotypes10,11, some of which may be amenable to TMC1 gene therapy.