Enrichment of differentially methylated probes in many CpG classes. Probes that were differentially methylated between blood and buccal samples (n = 69,174), or between blood and chorionic villus samples (n = 91,255), were assessed for enrichment in (A) Illumina and (B) HIL-annotated CpG classes. Enrichment was plotted as ‘percentage relative enrichment’, representing the enrichment of tDM probes relative to the total percentage of probes within each CpG class. Negative percentage relative enrichment indicates that tDM probes were depleted in the given probe-type category whereas positive percentage relative enrichment indicates that tDM probes were enriched in the given probe-type category. All enrichment analyses were significant with the exception of ICshore probes in the comparison of blood versus chorionic villi. HIL, high-density CpG island (HC), intermediate-density CpG island (IC) and non-island (LC); ICshore, intermediate-density CpG island shore; tDM, differentially methylated between tissues.