To test whether XBP1 expression makes a necessary contribution to in vivo beta cell compensation and diabetes resistance in obese mice, we crossed Xbp1flox/floxPdx1-CreER and ob/+ mice to generate Xbp1flox/floxPdx1-CreERob/ob (β-Xbp1−/−Ob) and Xbp1flox/floxPdx1-CreER (β-Xbp1−/−Wt) mice and control Xbp1+/+Pdx1-CreERob/ob (β-Xbp1+/+Ob) and Xbp1+/+Pdx1-CreER (β-Xbp1+/+Wt) mice. This evidence concerns the gene XBP1 and diabetes mellitus.