Halo2-tag is an engineered form of a Rhodococcus dehalogenase gene (DhaA) that forms a covalent bond with chloroalkane groups. A protein fused to Halo2-tag can react with a fluorescent compound carrying a chloroalkane group in vivo, resulting in rapid, irreversible and specific chemical tagging of the protein. Halo2-tag contains the mutations K175M, C176G, H272F and Y273L relative to the wild-type DhaA gene (PMID:18533659, PMID:23248739).