QR-421a aims to stop vision loss in people with Usher syndrome and retinitis pigmentosa due to mutation(s) in exon 13 of the USH2A gene. Mutations in exon 13 are a common cause of these conditions. The mutations cause a mistake in the USH2A RNA and therefore the cell cannot use the RNA to produce a working USH2A protein that is essential for vision.
QR-421a works by binding to the mutated USH2A RNA and exclude exon 13 from the RNA. This approach is known as exon skipping. The cells in the retina can then produce a slightly shorter but functional USH2A protein.