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. The gene discussed is MFN2; the disease is autosomal dominant optic atrophy.