Although the previous study reported polymorphisms at SLC22A1 associated with reduced metformin uptake, several clinical studies [51,89] and a meta-analyses of 5434 patients with T2DM across eight cohorts of the Metformin Genetics Consortium (MetGen) showed no significant association between SLC22A1 polymorphisms (R61C, M420del, combined genotype for R61C and M420del–number of reduced function alleles and rs622342) and glycemic response to metformin monotherapy [89]. This evidence concerns the gene SLC22A1 and type 2 diabetes mellitus.