The Halo-tag(Unk) entry in FlyBase represents an engineered form of a Rhodococcus dehalogenase gene (DhaA) that forms a covalent bond with chloroalkane groups, but where the precise form of the dehalogenase variant is unknown/unspecified. A protein fused to Halo-tag(Unk) can react with a fluorescent compound carrying a chloroalkane group in vivo, resulting in rapid, irreversible and specific chemical tagging of the protein.