DNAm sites were annotated to genes and CpG islands with the information provided in the Illumina manifest file, which is based on the UCSC RefGene and CpG island databases. Sites are annotated to genes if they are located within the gene body or up to 1,500 base pairs form the transcription start site. Sites are annotated to CpG islands if they are located within the boundaries of a CpG island, to a shore if they are located up to 2,000 base pairs from an island, or to a shelf if they are between 2,000 and 4,000 base pairs from an island. Frequency tables were used for recording the number of sites annotated to each feature category, and Chi-square tests were used for identifying different distributions across these annotation categories between all tested DNAm sites and the subset of sites considered for enrichment analysis (e.g., all DNAm sites with at least one significant mQTL).