The epithelium of the mammalian gastric body comprises multiple cell types replenished by a single stem cell. The adult conformation of cell lineages occurs well after birth; hence, study of genes regulating stem cell activity is facilitated by inducible systems for gene deletion. However, there is a potential pitfall involving the commonly used inducible Cre recombinase system to delete genes: we report here that induction of Cre using standard doses of tamoxifen led to marked spasmolytic polypeptide-expressing metaplasia of the stomach within days and profound atrophy of the entire epithelium with foci of hyperplasia by 2 wk even in the absence of loxP-flanked alleles. Cre induction caused genotoxicity with TdT-mediated dUTP nick-end labeling (TUNEL)-positive apoptosis (TUNEL-positive cells) and increased levels of DNA damage markers (gammaH2AX, p53, DDIT3, GADD45A). Although Cre was expressed globally by use of a chicken actin promoter, the effects were almost entirely stomach specific. Despite severe injury, a subset of mice showed near complete healing of the gastric mucosa 11-12 wk after Cre induction, suggesting substantial gastric regenerative capacity. Finally, we show that nongenotoxic doses of tamoxifen could be used to specifically delete loxP-flanked Bmpr1a, the receptor for bone morphogenetic protein 2, 4, and 7, causing antral polyps and marked antral-pyloric hyperplasia, consistent with previous reports on Bmpr1a. Together, the results show dose-dependent, potentially reversible sensitivity of the gastric mucosa to Cre genotoxicity. Thus we propose that tamoxifen induction of Cre could be used to induce genotoxic injury to study the regenerative capacity of the gastric epithelial stem cell.