In the 2011 November facts, I posted about Wolfram's Syndrome under the heading of different ways in which diabetics lose their sight. The gene that causes Wolfram Syndrome was the first gene discovered that pretty much always causes diabetes (instead of just increasing the risk). But Wolfram Syndrome is really rare; its estimated that about 1 in 400,000 people have it. The form of diabetes that Wolfram Syndrome causes used to be classified as type 1b- it's a diabetes caused by inability to make insulin as the islet cells atrophy- but its not autoimmune. The diabetes is usually diagnosed before the age of 10 and is usually insulin dependent.
Since the gene was discovered in 1998, it has been discovered that other mutations on the same gene, mutations that don't cause Wolfram Syndrome, dramatically increase the risk that a person will be diagnosed with diabetes, usually type 2 diabetes. Although the study of this gene's contribution to diabetes development is still in early days, it may contribute to as many as about 15% of cases of diabetes diagnosed as type 2.
I say diabetes diagnosed as type 2, rather than type 2 diabetes, because type 2 diabetes is currently defined as diabetes caused by an insulin secretory defect on a background of insulin resistance... and it's very plausible that this gene would cause diabetes in the absence of insulin resistance.