Editor—Seneca et al 1 reported a homoplasmic deletion of a T nucleotide in a 5T stretch (15 940-15 944 base pairs (bp)) of mitochondrial DNA (mtDNA) in two families associated with clinical and pathological findings of mt cytopathy. Although this deletion was homoplasmic and did not fulfil the classical criteria of pathological mutation, Seneca et al 1suggested that it was pathological, as they could not identify any other heteroplasmic mutations, deletions, or duplications in tRNA genes of mtDNA in these patients. However, this mutation was present not only in affected patients but also in asymptomatic relatives in both families. Therefore, this mutation does not cosegregate with the disease. It is difficult to confirm whether homoplasmic mutations are pathological, as was recently indicated by Chinneryet …