Comparison of the variability in methylation levels between CpG sites located within or outside CpG islands. The SDs for methylation levels measured for 1320 CpG sites across 401 ALL samples (529 320 observations) are shown on the horizontal axis. The vertical bars show the proportion of the observations in each bin of SD in methylation level. The graph shows that CpG sites outside CpG islands have greater variability in methylation levels between samples than CpG sites within CpG islands (P < .001).