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The mutation pattern of mitochondrial DNA (mtDNA) in mainland Chinese patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) has been rarely reported, though previous data suggested that the mutation pattern of MELAS could be different among geographically localized populations. We presented the results of comprehensive mtDNA mutation analysis in 92 unrelated Chinese patients with MELAS (85 with classic MELAS and 7 with MELAS/Leigh syndrome (LS) overlap syndrome). The mtDNA A3243G mutation was the most common causal genotype in this patient group (79/92 and 85.9%). The second common gene mutation was G13513A (7/92 and 7.6%). Additionally, we identified T10191C (p.S45P) in ND3, A11470C (p. K237N) in ND4, T13046C (p.M237T) in ND5 and a large-scale deletion (13025-13033:14417-14425) involving partial ND5 and ND6 subunits of complex I in one patient each. Among them, A11470C, T13046C and the single deletion were novel mutations. In summary, patients with mutations affecting mitochondrially encoded complex I (MTND) reached 12.0% (11/92) in this group. It is noteworthy that all seven patients with MELAS/LS overlap syndrome were associated with MTND mutations. Our data emphasize the important role of MTND mutations in the pathogenicity of MELAS, especially MELAS/LS overlap syndrome. [ABSTRACT FROM AUTHOR]