LAUSR

Tamoxifen is commonly used as a cancer treatment in humans and for inducing genetic alterations using Cre-lox mouse models in the research setting. However, the extent of tamoxifen off-target effects in animal research is underappreciated. Here, we report significant changes in cellular infiltration in Cre-recombinase-negative mice treated with tamoxifen intraperitoneally. These changes were noted in the lungs, which were characterized by the presence of alveolitis, vasculitis, and pleuritis. Despite significant immunological changes in response to tamoxifen treatment, clinical symptoms were not observed. This study provides a cautionary note that tamoxifen treatment alone leads to histologic alterations that may obscure research interpretations and further highlights the need for the development of alternative mouse models for inducible Cre-mediated deletion.