Studies were excluded from the current study based on the following criteria: studies published in languages other than English, studies that evaluated only combination therapies, i.e., TZDs plus metformin or TZDs plus metformin plus insulin, etc., studies that compared either TZDs or Metformin to other drugs, studies that included patients without T2DM, e.g., obese patients, and studies designed as systematic reviews and meta-analyses, letters to the editor, case reports, and abstracts without evidence of entire articles. This evidence concerns the gene INS and type 2 diabetes mellitus.