Model relating reprogramming factors and types of DNA methylation aberrations. A hypothetical genomic region showing four CpGs. White disk represent unmethylated CpG and black disk represent methylated CpG. Reprogramming with the Yamanaka factors leads to proper methylation but failure to demethylate, leading to enrichment of DNA demethylation aberrations in these iPSCs relative to ESCs. Similarly, reprogramming with the Thomson factors leads to proper demethylation but failure to methylate, leading to enrichment of DNA methylation aberrations in these iPSCs relative to ESCs.