A 59-year-old man received treatment for gastric cancer with nivolumab for 12 cycles over nine months. After discontinuation of the medication, four months later the patient presented to the emergency department with an elevated HbA1c of 10.6%, elevated blood glucose of 690 mg/dL (38.3 mmol/L), elevated anion gap of 15.2 mmol/L, weight loss, excessive thirst, and excessive urination. Urinalysis displayed ketone in urine and glucosuria. The patient was diagnosed with DKA and treated with lactated ringer rehydration therapy, bolus of insulin, and continuous insulin therapy. The gene discussed is INS; the disease is gastric cancer.