Box plots showing smoking-related differences in average methylation levels in different genomic regions. X-axis represents smoking habit (active smokers and non-smokers). Y-axis represents average methylation β values of CpG sites belonging to different genomic regions. Abbreviations and definitions for gene regions: TSS200, 0–200 bases upstream of the transcriptional start site; TSS1500, 200–1500 bases upstream of the TSS; 5′UTR, within the 5′ untranslated region, between the TSS and the ATG start site; Body, between the ATG and stop codon irrespective of the presence of introns, exons, TSS, or promoters; 3′UTR, between the stop codon and poly A signal. Definitions for CpG islands: Shores, 0–2 kb from CpG island; Shelves, 2–4 kb from CpG island.