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Background: Evidence hints at the ability of β-cells to emerge from non-β-cells upon genetic or pharmacological interventions. However, their quantitative contributions to the process of autonomous β-cell regeneration without genetic or pharmacological manipulations remain to be determined. Methods & results: Using PANIC-ATTAC mice, a model of titratable, acute β-cell apoptosis capable of autonomous, and effective islet mass regeneration, we demonstrate that an extended washout of residual tamoxifen activity is crucial for β-cell lineage tracing studies using the tamoxifen-inducible Cre/loxP systems. We further establish a doxycycline-inducible system to label different cell types in the mouse pancreas and pursued a highly quantitative assessment to trace adult β-cells after various metabolic challenges. Beyond proliferation of pre-existing β-cells, non-β-cells contribute significantly to the post-challenge regenerated β-cell pool. α-cell trans-differentiation is the predominant mechanism upon post-apoptosis regeneration and multiparity. No contributions from exocrine acinar cells were observed. During diet-induced obesity, about 25% of α-cells arise de novo from β-cells. Ectopic expression of Nkx6.1 promotes α-to-β conversion and insulin production. Conclusions: We identify the origins and fates of adult β-cells upon post-challenge upon autonomous regeneration of islet mass and establish the quantitative contributions of the different cell types using a lineage tracing system with high temporal resolution. Author Video: Author Video Watch what authors say about their articles Keywords: Lineage tracing, Adult β-cell origins, Nkx6.1, Tamoxifen artifacts