The study hypothesis is that people who respond well to sulphonylureas have a different underlying cause for their diabetes than people who respond poorly to this medication. We are using two approaches to study this. In one approach we look at people who have previously responded well or poorly, confirm this by rechallenging them with a sulphonylurea drug, and then looking at how well they produce insulin in response to glucose and an intravenous sulphonylurea called tolbutamide. The second approach identifies people with a certain genetic predisposition to diabetes (due to changes in the TCF7L2 gene) and then looks at how well they respond to sulphonylurea medication.