TMP-tag is an engineered form of the Escherichia coli DHFR (dihydrofolate reductase) gene that forms a covalent bond with trimethoprin derivatives. A protein fused to TMP-tag can react with a fluorescent trimethoprin derivative in vivo, resulting in rapid, irreversible and specific chemical tagging of the protein (see PMID:15782216 and PMID:22873118).