To examine whether defective muscle glycogen synthase (GYS1) expression is associated with impaired glycogen synthesis in type 2 diabetes and whether the defect is inherited or acquired, we measured GYS1 gene expression and enzyme activity in muscle biopsies taken before and after an insulin clamp in 12 monozygotic twin pairs discordant for type 2 diabetes and in 12 matched control subjects. The effect of insulin on GYS1 fractional activity, when expressed as the increment over the basal values, was significantly impaired in diabetic (15.7±3.3%; P＜0.01), but not in nondiabetic (23.7±1.8%; P =NS) twins compared with that in control subjects (28.1±2.3%). Insulin increased GYS1 messenger ribonucleic acid (mRNA) expression in control subjects (from 0.14±0.02 to 1.74±0.10 relative units; P, 0.01) and in nondiabetic (from 0.24±0.05 to 1.81±0.16 relative units; P＜0.01) and diabetic (from 0.20±0.07 to 1.08±0.14 relative units; P＜0.01) twins. The effect of insulin on GYS1 expression was, however, significantly reduced in the diabetic (P, 0.003), but not in the nondiabetic, twins compared with that in control subjects. The postclamp GYS1 mRNA levels correlated strongly with the hemoglobin A1c levels (r=-0.61; P＜0.001). Despite the decrease in postclamp GYS1 mRNA levels, the GYS1 protein levels were not decreased in the diabetic twins compared with those in the control subjects (2.10±0.46 vs. 2.10±0.34 relative units; P 5 NS). We conclude that 1) insulin stimulates GYS1 mRNA expression; and 2) impaired stimulation of GYS1 gene expression by insulin in patients with type 2 diabetes is acquired and most likely is secondary to chronic hyperglycemia.