A 73-year-old patient diagnosed with metastatic melanoma was treated with vemurafenib and cobimetinib initially but with the progression of disease a PD-1 inhibitor nivolumab was added. After three cycles, the patient developed abdominal pain, vomiting, asthenia, polyuria, and polydipsia. The lab tests showed glycemia of 27.78 mmol/L (500 mg/dL), ketonuria, glucosuria, bicarbonate of 18 mmol/L, and Hb1Ac of 8.8%. The patient was diagnosed with type 1 diabetes with acute renal failure and treated with insulin. The patient then began therapy on nivolumab without any episode of hyperglycemia. This evidence concerns the gene INS and type 1 diabetes mellitus.