MT-ND4 and Leber hereditary optic neuropathy: The most prevalent LHON-causing mutation is mt.11778G>A in the mitochondrial <i>MT-ND4</i> gene, which encodes a critical subunit of complex I. Allotopic expression, a promising gene therapy strategy, aims to deliver a functional nuclear version of <i>ND4</i> into the cell nucleus and target the resulting protein to the mitochondria.