of the prefrontal cortex, during fetal compared with postnatal life (Figure 1A). While the Illumina 450k largely targets CpGs in and around gene promoter regions (islands, shores and shelves)17, almost every annotated gene (17,300/19,771 via UCSC knownGene hg19 table, 87.5%) contained at least one differentially methylated CpG within 5kb. These effects were further evident across the entire array - using principal component analysis, the first principal component, explaining 55.6% of variability in the data, strongly correlated with pre- versus post-natal life (Supplementary Figure 1).
