DNA methylation was assayed using the Illumina Infinium HumanMethylation27 BeadChips (Illumina Inc., San Diego, CA) platform; the assay was performed at the Genomics Core Facility at Northwestern University. 27,578 CpG sites were probed, almost all of which (approximately 97.7%) are within 1500 bp of a transcription start site (TSS). Methylation level at each interrogated CpG site was defined as: methylated probe intensity/(sum of methylated and unmethylated probe intensities) Therefore, methylation level is a quantitative trait whose values range from 0 to 1.
