It's kind of amazing. The issue is that the cells have a dysfunctional copy of the OTOH gene, which leads to production of a dysfunctional otoferlin protein tha

It's kind of amazing. The issue is that the cells have a dysfunctional copy of the OTOH gene, which leads to production of a dysfunctional otoferlin protein that does not work. There are virii that inject their DNA into cells. They modify one of these to make them inject a working copy of the OTOH gene into the cell. Cells have a mechanism where if there is a gene floating around nearby it will sometimes pick it up and integrate it. This is the process that is exploited in gene therapy to change individual cells to produce the correct protein. If enough cells produce the correct protein, there will be enough to have the desired effects. It's really unbelievably cool. I wish there were more appreciation for how these things work.