Single large deletions of mitochondrial DNA are found in the muscle of about 40% of patients with mitochondrial myopathies, and are detectable in both blood and muscle in Pearson syndrome. In mitochondrial myopathies, there is a close association between the presence of deletions and involvement of extra-ocular muscles, together with other features of the Kearns-Sayre syndrome. Deletions appear to arise as fresh mutations in the vast majority of patients and are often flanked by direct repeats up to 13 nucleotides in length. They should affect translation of all mitochondrially encoded components of the respiratory chain, but there is evidence to suggest that intramitochondrial complementation occurs in some cases.