In view of great variability in plasma glucose levels despite adherence to treatment, C-peptide levels showing insulin secretion (which is not compatible with T1DM), absence of GAD antibodies, and, most importantly, a strong family history of diabetes in the mother and maternal grandmother, the diagnosis of MODY was suspected and confirmed as HNF1A MODY by molecular testing. The gene discussed is GAD1; the disease is type 1 diabetes mellitus.