A 58-year-old patient diagnosed with metastatic melanoma was treated with vemurafenib and ipilimumab, for 16 months resulting in a stable disease. Two years following treatment, the patient developed new intra-abdominal, subcutaneous, and liver metastasis and was started on PD-1 therapy with pembrolizumab. After receiving 17 cycles of therapy, they presented with a plasma glucose of 408 mg/dL (22.7 mmol/L) and elevated HbA1c of 9.7% resulting in the diagnosis of insulin-dependent diabetes. The patient was treated with insulin, and blood glucose levels neutralized. This evidence concerns the gene INS and type 1 diabetes mellitus.