We reported that a group of 19 patients with non-arteritic ischaemic optic neuropathy (NAION) had an increased number of mitochondrial (mt) DNA nucleotide changes1 and significantly increased relative mtDNA content compared to age and sex matched controls.2 We were pleased that Carelli and Sadun thought that our most recent manuscript in BJO warranted an editorial response.3 In this letter, we clarify data about mtDNA nucleotide changes and relative mtDNA content in NAION. Referring to our first report, they stated that “… most of the polymorphisms found in NAION patients are ancient, well known mtDNA variants defining specific haplogroups (mtDNA maternal lineages) …