Col1α2-Cre-mediated recombination occurs in various cell types due to Cre expression in epiblasts.

The Cre-LoxP system has been commonly used for cell-specific genetic manipulation. However, many Cre strains exhibit excision activity in unexpected cell types or tissues. Therefore, it is important to identify the cell types in which recombination takes place. Fibroblasts are a cell type that is inadequately defined due to a lack of specific markers to detect the entire cell lineage. Here, we investigated the Cre recombination induced by Col1α2-iCre, one of the most common fibroblast-mesenchymal Cre driver lines, by using a double-fluorescent Cre reporter line in which GFP is expressed when recombination occurs. Our results indicated that Col1α2-iCre activity was more extensive across cell types than previously reported: Col1α2-iCre-mediated recombination was found in not only cells of mesenchymal origin but also those of other lineages, including haematopoietic cells, myocardial cells, lung and intestinal epithelial cells, and neural cells. In addition, study of embryos revealed that recombination by Col1α2-iCre was observed in the early developmental stage before gastrulation in epiblasts, which would account for the recombination across various cell types in adult mice. These results offer more insights into the activity of Col1α2-iCre and suggest that experimental results obtained using Col1α2-iCre should be carefully interpreted.