Further supporting the AHRR locus and probe cg05575921 in particular is the variety of studies showing associations. As with F2RL3, studies have replicated the association in samples including men [28,65], women [27,50,52,53,54,61] and both sexes combined [49,50,51,53,66,67,68,69,70,71,72]. The association is present in smokers as young as age 19 [66] through age 60 and later [48]. The signal is robust across multiple ethnic groups [27,28,54], and different windows of exposure [67,69]. The association has excellent quantitative effects, with a high average delta beta of over 20% in many of the studies of older smokers [28,49,53], and effects of both cumulative pack years smoked and cessation time on methylation observable in former smokers [49,54,66]. Many studies have included additional experiments to bolster the validity of findings at this locus, including confirmatory bisulfite pyrosequencing [49,50,52], measurement of AHRR gene expression [27,50,51,53,54,62], statistical techniques to control for GxMeth effects [66] and for peripheral blood cell mixture [27,28,51,53,54,67,68], and replication of findings in additional samples [49,53].
