Therefore, 25 eligible papers [2,53,55-71,74,75,77,81-83], including 36 studies (multiple studies performed in one paper in particular: 3 studies in paper of Grant et al [2], Horikoshi et al [62], Humphries et al [67], and Miyake et al [82]; 2 studies in paper of Scott et al [74], Sladek et al [75], and Cauchi et al [77]), examining the association between TCF7L2 gene polymorphisms and T2DM were identified by the inclusion criteria and all of them were written in English (Table 1, Additional file 1). Here, TCF7L2 is linked to type 2 diabetes mellitus.