LHON (OMIM 535000) is the most common of the primary mitochondrial DNA (mtDNA) disorders, and the minimum prevalence has been estimated at 1 in 31 000 in the North of the United Kingdom (Figure 1).12 Comparable prevalence figures of 1 in 39 000 and 1 in 50 000 have been reported in epidemiological studies from the Netherlands and Finland, respectively.13, 14 The majority of cases (90%) are due to one of three mtDNA point mutations: m.3460G>A (MTND1), m.11778G>A (MTND4), and m.14484T>C (MTND6), which affect key complex I subunits of the mitochondrial respiratory chain. This evidence concerns the gene MT-ND4 and Leber hereditary optic neuropathy.