Figure 4 Mean amount of wild‐type mitochondrial DNA (mtDNA) in 11 subjects with the 3243A→G mutation on two occasions. y Axis: wild‐type mtDNA (mtDNA copy number) was determined by real‐time polymerase chain reaction (PCR) in triplicate using iQ Sybr Green on the BioRad ICycler to determine the total amount of mtDNA. The wild‐type mtDNA was calculated from the mean percentage determined by last cycle fluorescent PCR shown in fig 2. x Axis: age of the subject (years) at the time of sampling. Each patient has a different symbol and colour, with the serial measurements connected by a straight line and using the same colour code as fig 2.