The SNAP-tag(Unk) entry in FlyBase represents an engineered form of the human AGT (O[6]-alkylguanine-DNA alkyltransferase) gene that forms a covalent bond with benzylguanine derivatives, but where the precise engineered variant is unknown/unspecified. A protein fused to SNAP-tag(Unk) can react with a fluorescent benzylguanine derivative in vivo, resulting in rapid, irreversible and specific chemical tagging of the protein.