XBP1 and obesity due to melanocortin 4 receptor deficiency: In contrast, the incidence of TUNEL-positive cells, although unchanged in β-Xbp1−/−Wt mice, was significantly increased by threefold in β-Xbp1−/−Ob mice (Fig. 3l,m), suggesting that the absence of XBP1 increases the rate of beta cell apoptosis under conditions of obesity.