The patient with ketosis-prone type 2 diabetes is more often male, middle-aged, overweight, or modestly obese (type 1 obesity); has a family history of type 2 diabetes; presents with new-onset severe hyperglycemia and ketosis or frank diabetic ketoacidosis; and is GAD and islet cell antibody negative. This evidence concerns the gene GAD1 and type 2 diabetes mellitus.