By precisely inserting the P2A-CreERT2 expression cassette at the termination codon site at the end of the mouse Col1a2 coding sequence, the Col1a2-P2A-CreERT2 knock-in allele was successfully constructed. In this model, Col1a2 and CreERT2 are co-expressed from the same open reading frame; during translation, P2A induces ribosomal skipping, allowing the upstream Col1a2 protein and the downstream CreERT2 recombinase to form independent protein products without producing an intact Col1a2-CreERT2 fusion protein. It should be noted that the action of P2A is essentially a peptide bond skipping event during co-translation rather than classical protease-mediated post-translational cleavage, and both the upstream protein and downstream CreERT2 may retain a small number of P2A-related amino acid residues [1].

The Col1a2 gene encodes the type I collagen $\alpha2$ chain, which is an essential component of the type I collagen triple helix. Type I collagen is predominantly distributed in the extracellular matrix of skin, tendons, bone, heart, and other connective tissues, playing a crucial role in mechanical tissue support, cell adhesion, migration, and injury repair. Because Col1a2 regulatory elements drive gene expression in diverse fibroblast populations, the Col1a2-CreERT2 system has been utilized for fibroblast labeling, lineage tracing, and the study of fibrosis-related gene functions [2,3]. CreERT2 is an inducible recombinase composed of the Cre recombinase fused to a mutated estrogen receptor ligand-binding domain. In the absence of tamoxifen, CreERT2 remains predominantly cytoplasmic and unable to efficiently enter the nucleus, whereas tamoxifen administration promotes its nuclear translocation, thereby mediating loxP site recombination in mice carrying floxed alleles [4]. Consequently, when Col1a2-P2A-CreERT2 mice are crossed with floxed mice and treated with tamoxifen, temporally controlled conditional gene knockout, knock-in, or lineage tracing can be achieved in Col1a2-positive fibroblasts and their progeny. This provides a valuable genetic tool for investigating fibroblast functions, tissue injury repair, organ fibrosis, vascular remodeling, and tumor stromal responses across skin, heart, lungs, kidneys, and other tissues.