Gene therapy is the process of insertion, alteration, and/or removal of targeted genes to and from an individual’s cells to treat medical conditions. Typically, a copy of the coding zone of a target gene is carried by vectors to targeted tissues, organs, and cells. Genetic manipulation at the genome level is not considered to reduce the associated risks. Local gene transfection to a limited region is one of the most convenient ways to enable such control. The cochlea is traditionally considered as an ideal organ for local gene therapy because of its anatomical isolation and the fluid streaming internally. At least theoretically, the isolation may limit the risk of side effects that may occur in other tissues and organs, and fluid movement may facilitate transportation of the targeted gene. However, the effectiveness of this isolation has not been comprehensively evaluated in the situation of virus mediated cochlear gene transfection. Genetic defects play a significant role in sensorineural hearing loss, one of the most common neurological disorders. Approximately 70% of genetic hearing loss disorders are nonsyndromic hearing loss, involving more than 100 identified genetic loci. In those cases, the deficit is selective to inner ears and may be corrected by local gene therapy in this isolated organ. Various vectors to transfer target genetic material into target tissue or cells have been investigated. At present, viral vectors are considered to have the best transfection efficiencies among vectors. Within this catalog, adenovirus, adeno-associated virus, lentivirus, and herpes SAR131675 simplex virus have been investigated in the inner ears of mammals. Among these, AAVs are favored because they are not pathogenic or ototoxic, can transfect most cell types including neurons in the cochlea, and provide long-lasting expression of transfected genes. Moreover, different approaches for vector delivery have been evaluated. In a recent paper, we reported the use of a newly developed adenoassociated virus with mutations of surface-exposed tyrosine residues, in combination with digestion of the round window membrane to achieve inner ear gene transfection in guinea pigs. The result was satisfactory and the method was minimally invasive. Mouse models are extremely important and useful for the study of genetic hearing loss for several reasons, which have been extensively reviewed. Briefly, their gene map is nearly complete, various genetic manipulations for hearing loss are available, and most mouse models faithfully mimic hearing impairments in humans. The technology used to generate genetically modified mice has been improved, and the methods are tractable and reproducible across laboratories. Thus, it is reasonable that cochlear gene transfection conducted in laboratory mice should be the first step in research to correct genetic deficits in hearing loss. In many cases, genetic hearing losses develop in the early stages of development and involve deformation of the auditory sensory organ. This deformation cannot be corrected after it is established. Therefore, genetic interference to correct the deficit must be administered early. Early intervention presents a challenge in the mouse model, because the small size of the neonatal cochlea makes surgical manipulation extremely difficult. Potential rejection of the pup by the mother after surgery presents an additional problem. To our knowledge, only one study on cochlear gene transfection in neonatal mice has been reported. In that brief report, cochlear transfection was conducted using AAV and AV. However, the cell transfection rate has not been quantitatively reported.