A 30-year-old female previously diagnosed with C-peptide (CP)-positive, autoantibody-negative type 1 diabetes mellitus (T1DM) at 19 years old presented to the clinic at age 28 for management of diabetes mellitus (DM) that had previously been controlled by insulin since diagnosis. Laboratory results from May 2011 showed low-normal C-peptide of 1 ng/mL (normal range: 0.8-4 ng/mL) with no corresponding glucose, glutamic acid decarboxylase (GAD)-65 antibody (GADA) of <1 U/mL (N<1.1 U/mL at the time of laboratory draw), and HbA1c of 6.4%. This evidence concerns the gene CP and diabetes mellitus.