The two top CpG sites, cg24908166 and cg12324353, were annotated to TERT (Table 4). The two CpG sites were positively correlated with each other (r = 0.27, p < 0.001) and were located within 1 kb from each other in an intron of TERT. Two other CpG sites cg05603985 and cg09469355 were located within 1 kb from each other in the first exon and intron of SKI. Methylation of these two sites was positively correlated (r = 0.57, p < 0.001). CpG sites cg09397246 and cg26405020 were located within 1500 base pairs from the transcription start site of FES. These sites were within two base pairs from each other and had a positive correlation (r = 0.88, p < 0.001). The other significant hits were annotated to SARS, GNGT2, SMG6, TOM1L2, SIPA1, MRAS, CDKN1A, LRRC2 and RPH3A. The beta-value distributions for all identified CpG sites stratified by the three smoking categories can be found in Additional file 2: Figure S1.Table 4Characteristics of significant CpG sitesAverage beta-values (SD)CpG siteGenePositiona PlacementCurrent smokersFormer smokersNever smokerscg24908166TERT5:1,268,800Body0.893 (0.031)0.900 (0.029)0.908 (0.024)cg12324353TERT5:1,269,197Body0.846 (0.027)0.851 (0.027)0.855 (0.024)cg03725309SARS1:109,757,585Body0.292 (0.057)0.297 (0.060)0.311
