Editor—Disorders of oxidative phosphorylation are highly heterogeneous from both a clinical and a genetic point of view. The nuclear as well as the mitochondrial genomes contain genes that are necessary for respiratory chain function. Consequently, different modes of inheritance are encountered in disorders of oxidative phosphorylation.

Single large scale deletions of mitochondrial DNA (mtDNA) usually occur in sporadic cases.1 However, multiple deletions of mtDNA also occur in autosomal dominant disorders.2 These deletions are generated de novo as somatic mutations in each affected subject. The nuclear gene defects predisposing to secondary mtDNA deletions in these patients remain unknown.

The disorder discovered by Zeviani et al 2 was later found in several families and was called autosomal dominant progressive external ophthalmoplegia (ADPEO),3 as ptosis and external ophthalmoplegia are the major clinical findings.4-7 More generalised weakness of the skeletal muscles and sudden unexpected death are also common clinical features.4-7 Additional features vary among different families.4 6-8

Linkage analysis provided direct evidence for genetic heterogeneity …