Recent discoveries related to deafness genes have considerably increased our understanding of the molecular mechanism of hearing. The cochlear and vestibular endorgans consist of various cell types, with each cell expressing the genes necessary for its function and producing the proteins necessary for hearing and balance. Pathogenic variants in these genes cause hearing loss due to a deficiency or alteration in the proteins necessary for hearing. The inner hair cells (IHCs) are essential for the conversion of mechanical movements to electric signals, and the outer hair cells (OHCs) act as amplifiers of auditory sound signals. The IHCs and OHCs express a variety of deafness-related genes, particular stereocilia-related genes, reflecting the distinct and important roles of both types of hair cells in sound transduction mechanisms. Supporting cells, including pillar cells, inner phalangeal cells, border cells, and Deiters’ cells, also play crucial roles in normal hearing function, particularly in the glutamate-glutamine cycle and potassium ion recycling system, which are essential for hair cell transduction. Genes expressed on the spiral prominence are important for maintaining the anion balance. The stria vascularis and spiral ligament express genes necessary to the energy source for effective mechanoelectric transduction. Further, the tectorial membrane- and basilar membrane-related genes are important for sufficient amplification of sound stimuli. The coordinated action of these genes is necessary for maintaining normal function, and the impairment of gene function by pathogenic variants leads to hearing loss.