A major portion of biochemical studies [69,70,71,72,73,74] has revealed that patients with type 2 diabetes and prediabetes exhibit the increased blood levels of γ-glutamyltransferase (GGT), and it has been suggested that an increase in GGT concentration represents a sensitive and early biomarker for the development of diabetes. This evidence concerns the gene GGT1 and type 2 diabetes mellitus.