The authors previously established a transgenic mouse line in the type 1 diabetes model, NOD mouse, in which thioredoxin (TRX), a redox protein, is overexpressed in pancreatic β cells, and found that TRX overexpression slows the progression of type 1 diabetes. Recent reports on type 2 diabetes suggest that oxidative stress also degrades the function of β cells. To elucidate whether TRX overexpression can prevent progressive β cell failure from oxidative stress in type 2 diabetes, the authors transferred the TRX transgene from the NOD mouse onto a mouse model of type 2 diabetes, the db/db mouse. The progression of hyperglycemia and the reduction of body weight gain and insulin content of the db/db mouse were significantly suppressed by the TRX expression. Furthermore, TRX suppressed the reduction of Pdx-1 and MafA expression in the β cells, which may be one of the cellular mechanisms for protecting β cells from losing their insulin-secreting capacity. These results showed that TRX can protect β cells from destruction not only in type 1 but also in type 2 diabetes, and that they provide evidence that oxidative stress plays a crucial role in the deterioration of β cell function during the progression of type 2 diabetes.