Examination of twin-pair discordance with age. The absolute within-pair discordance values were calculated for each twin pair and the change in discordance over time was assessed. (a) Scatterplots of methylation levels (β values) for six twin pairs versus their co-twin visualized at birth and at 18 months showing two examples each of pairs exhibiting within-pair drift, convergence and stability between birth and 18 months of age as defined in the text. Points shown in red represent those with an absolute within-pair discordance value of >0.2 (20%). The number of discordant probes is shown in the bottom right corner of each plot. MZ, monozygotic; DZ, dizygotic; MC, monochorionic; DC, dichorionic. (b) Euclidean distance of within-pair discordances plotted for each twin pair at birth and 18 months. Within-pair discordance increases in those pairs that drift and decreases in those that converge; stable pairs remain with similar values. (c) Distribution of the delta discordance values defined as absolute discordance at 18 months minus absolute discordance at birth. (d) Boxplot comparison of the change in beta values with age (deltabeta) versus change in discordance values with age (deltadiscordance).