(A) Application of doxycycline (Dox) to the drinking water of Insulin-rtTA;TET-DTA mice induces the expression of DTA specifically in β cells, causing apoptotic β cell death. (B) Normal islet morphology in a 4-week-old double-transgenic mouse untreated with doxycycline. (C) Normal blood glucose levels (measured during the day) in 3-week-old mice in the absence of doxycycline. Untreated transgenic mice at 1.5 years had blood glucose levels indistinguishable from those of age-matched wild-type mice (not shown). (D) In the absence of doxycycline, 4-week-old double-transgenic mice had normal fasting blood glucose levels and glucose tolerance. Values are mean ± SD (n = 5–7). Single-transgenic TET-DTA littermates are referred to as wild-type. (E) β Cell specificity of ablation. Four-week-old mice were treated with doxycycline for 48 hours, then sacrificed and assessed for apoptotic cell death. Extensive β cell apoptosis was detected by TUNEL staining in double-transgenic mice. No apoptosis was seen in non-β cells in islets or in the exocrine pancreas. Doxycycline-treated single-transgenic littermates showed no sign of apoptosis. (F) Quantification of β cell apoptosis detected 48 hours after the administration of doxycycline to 4-week-old mice. Values are mean ± SD from 5-week-old mice exposed to doxycycline for 48 hours before sacrifice (n = 3 per group). For each mouse, about 2,000 β cells were counted. Scale bars: 100 μm.