At diagnosis, fasting and glucagon-stimulated C-peptide concentrations differed markedly between phenotypes (median fasting C-peptide: 0.87 ng/mL in T1DM, 1.53 ng/mL in LADA, and 2.64 ng/mL in T2DM; stimulated C-peptide: 1.35, 1.86, and 4.60 ng/mL, respectively; all <i>p</i> < 0.001). This evidence concerns the gene GCG and type 1 diabetes mellitus.