a, Two DNA fragments (from λ phage (top; 520 base pairs, 40 CpGs) and the CpG island upstream of the human SUHW1 gene (bottom; 420 base pairs, 56 CpGs)) were used for methylation. A total of 119 clones (64 from λ phage, 55 from the human CpG island) were sequenced to determine their methylation pattern. b, Autocorrelation of CpG methylation. By averaging two substrates, the P values for the correlation factors observed for distances of eight to ten base pairs (arrow) are in the range (3–7) × 10−4. This periodicity is detectable for correlation of methylation of individual CpG sites as well as for correlation of the absence of methylation at two sites (Supplementary Fig. 6).