Editor—Severe, isolated, and generalised deficiency of complex IV (cytochrome c oxidase, COX) can result in Leigh syndrome (LS) (MIM 256000), an early onset mitochondrial disorder characterised by rapidly progressive, symmetrical degeneration of the brain stem, diencephalon, and basal ganglia.1 2 SURF-1, a gene located on chromosome 9q34, has recently been identified as the gene responsible for numerous cases of LS^COX.3 4

SURF-1 associated LS^COX is usually inherited as an autosomal recessive trait. We report here a homozygous loss of function mutation ofSURF-1 in two monozygotic LS^COXfemale twins, owing to uniparental disomy of two almost identical maternal chromosomes 9.

The probands were born to non-consanguineous parents at 33 weeks of gestation by caesarean section. The mother was 46 years old. The pregnancy was uneventful until the 24th week, when persistent uterine contractions ensued. Two older sibs of the probands are alive and healthy. The family history was negative for neurological or metabolic disorders. Birth weight and body length were <3rd centile and the Apgar score was 1-8 in both patients. Growth rate and psychomotor development were regular during the first 8 months of life. During the following months the patients developed a rapidly progressive clinical syndrome characterised by failure to thrive, psychomotor regression, hypotonia, ophthalmoparesis, mild bilateral optic atrophy, and ataxia. At 18 months both patients had mild lactic acidosis. MRI …