Mutations in the fusion genes OPA1 and MFN2 have been associated with dominant optic atrophy (Alexander et al, 2000; Delettre et al, 2000) and CMT Type 2A (Zuchner et al, 2004), respectively, but the reasons for these genotype–phenotype correlations have remained an enduring mystery. This evidence concerns the gene OPA1 and autosomal dominant optic atrophy.