Mitochondrial DNA (mtDNA) depletion syndromes (MDDS) are a group of clinically heterogeneous autosomal recessive disorders characterised by a severe quantitative reduction of total mtDNA, the genetic material present exclusively within mitochondria. mtDNA is a 16.5 kb circular genome, encoding 13 subunits of the respiratory chain and 24 RNA molecules necessary for the intramitochondrial translation of these 13 proteins. Unlike nuclear DNA, where every cell contains two copies of each gene (one copy from each parent), mtDNA is a multicopy genome, each cell containing thousands of copies. mtDNA depletion has been defined as a residual mtDNA copy number of< 30% compared with age-matched controls, 1 2 but mtDNA levels are often< 10%, and sometimes as little as 1–2%, of controls, particularly in the hepatocerebral form of the disease. Recognised clinical presentations of MDDS include early-onset hepatocerebral disease overlapping with Alpers syndrome, isolated myopathy, encephalomyopathy and the MNGIE (mitochondrial neurogastrointestinal encephalomyopathy) syndrome (table 1).