Auditory neuropathy is a relatively rare form of hearing loss in which everything in the ear is fully functional but sounds are not transmitted properly to the auditory nerve and the brain. Hearing aids and cochlear implants both have limited success with people who have auditory neuropathy.
According to this article, scientists at the University of Michigan Medical Center have identified a gene mutation which can cause this form of deafness. The University of Michigan's website has some more information on their findings. They were able to genetically engineer fruit flies that had an overabundance of a certain kind of protein in their auditory organs, then measured the fruit flies' response to sounds.
According to a professor at the university, "The approach we used here of combining genetic inheritance with functional information can be applied to identify the culprit genes in many other rare genetic diseases that have so far been impossible to nail down."