Prior studies of gene-smoking interaction for T2D risk have used a candidate gene approach, focusing on loci associated either with smoking behavior, such as CYP2A6 [74] or the nicotinic acetylcholine receptor gene (CHRNA4) [75], or with T2D and other metabolic traits [76], including HNF1A [77] and APOC3 [78]. Here, CHRNA4 is linked to type 2 diabetes mellitus.