Nucleotides, base-pairing definition

Molecular analysis of mtDNA from muscle biopsies often provides a definitive diagnosis of MELAS. Individuals with more severe clinical manifestations of MELAS generally have greater than 80% mutant mtDNA in postmitotic tissues such as muscle. In approximately 80% of MELAS cases, the responsible mutation is an A —> G base substitution at nucleotide position 3243. A smaller subset of MELAS patients (7.5%) possesses a different point mutation, aT C transition at nucleotide position 3271. At least eight additional point mutations and one four-base pair deletion mutation also have been described. 

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