In a case-control design, we determined mRNA expression, protein abundance, and phosphorylation of key UPR markers in skeletal muscle biopsies obtained from patients with T2D, matched to glucose-tolerant individuals with obesity and lean individuals, before and after 4-h insulin infusion during a hyperinsulinemic-euglycemic clamp. This evidence concerns the gene INS and obesity due to melanocortin 4 receptor deficiency.