Figure 1. Gene therapy by cochlear implant. Cochlear implants are multielectrode arrays that treat severe forms of hearing loss caused by the absence of mechanosensory hair cells. Pinyon et al. adapted a cochlear implant marketed by Cochlear Ltd. to deliver a plasmid encoding brain-derived neurotrophic factor (BDNF), a growth factor that promotes sensory neuron growth. Activation of selected electrodes along the implanted array led to localized plasmid delivery to cochlear tissue near the activated electrodes. BDNF production and neurite outgrowth occurred near the regions that received plasmid but not in other parts of the cochlea.