Loss of pancreatic islet b-cell mass and b-cell dysfunction are central in the development of type 2 diabetes (T2DM). We recently showed that mature human insulincontaining b-cells can convert into glucagon-containing a-cells ex vivo. This loss of b-cell identity was characterized by the presence of b-cell transcription factors (Nkx6.1, Pdx1) in glucagon + cells. Here, we investigated whether the loss of b-cell identity also occurs in vivo, and whether it is related to the presence of (pre)diabetes in humans and nonhuman primates. We observed an eight times increased frequency of insulin + cells coexpressing glucagon in donors with diabetes. Up to 5% of the cells that were Nkx6.1 + but insulin 2 coexpressed glucagon, which represents a five times increased frequency compared with the control group. This increase in bihormonal and Nkx6.1 + glucagon + insulin 2 cells was also found in islets of diabetic macaques. The higher proportion of bihormonal cells and Nkx6.1 + glucagon + insulin 2 cells in macaques and humans with diabetes was correlated with the presence and extent of islet amyloidosis. These data indicate that the loss of b-cell identity occurs in T2DM and could contribute to the decrease of functional b-cell mass. Maintenance of b-cell identity is a potential novel strategy to preserve b-cell function in diabetes.Loss of pancreatic b-cell mass and b-cell dysfunction are central in the development of type 2 diabetes (T2DM) and, in combination with peripheral insulin resistance, lead to hyperglycemia (1). Whereas b-cells, on the one hand, fail to properly secrete insulin at a given glucose level, there is also a progressive decline in the number of b-cells (2,3). Loss of b-cell mass has been ascribed to increased apoptosis in T2DM (4). In patients with T2DM, b-cell mass can be up to 40-60% lower than in healthy control subjects (4-6). In addition, abnormal function of glucagon-producing a-cells leading to hyperglucagonemia is associated with T2DM (7). b-cell dedifferentiation and subsequent transition to other islet cell types were suggested as an alternative explanation for the loss of functional b-cell mass in mice (8,9). In this concept, b-cells lose insulin content and insulin secretory capacity followed by the production of other endocrine hormones such as glucagon (8). We recently showed (10) that loss of b-cell identity with the conversion of b-cells into glucagon-containing a-cells can occur in human pancreatic islets ex vivo.A number of transcription factors have been identified to be essential for the development and maintenance of functional b-cells (11,12). Recent reports (13,14) indicate that a selective loss of transcription factors MafA, Nkx6.1, and Pdx1 is associated with b-cell dysfunction and T2DM. Chronic hyperglycemia in rats is accompanied by the loss of b-cell transcription factors (15). Moreover, mouse b-cells that genetically lack FOXO1 can dedifferentiate in vivo under conditions of metabolic stress and subsequently can convert (or transdifferentiate) into glucagonproducing a-cells...