Inter-individual variation of DNA methylation. (a) DNA methylation levels of all amplicons studied in blood from ten old (1 to 10) and ten young (11 to 20) individuals. The data were sorted by the average level of DNA methylation among amplicons and individuals. Old individuals are represented by black numbers, young individuals by red numbers. The continuous yellow to blue color code represents the DNA methylation level of the PCR products. (b) Average DNA methylation level of all 16 amplicons among 20 individuals (1 to 20). The error bars indicate the standard deviation among the samples. Amplicon 262 was split into 262_L (left part) and 262_R (right part) because of the different methylation observed in these two parts. Green color labeled amplicons showed a maximal methylation difference between individuals of more than 50%; orange labels indicate amplicons with >30% methylation difference. (c) DNA methylation pattern of amplicon 23_1 in individuals 1 to 20. Each column represents a single CpG site. Each row corresponds to one individual.