DT-A(Su+) corresponds to a mutated version of the catalytic A subunit encoded by the Corynephage beta Diphtheria toxin (DT) gene (UniProtKB:P00588), into which an amber stop mutation (TAT to TAG) has been introduced at residue 28 (FBrf0059294). The mutation is designed to allow expression of the toxin to be controlled by the presence or absence of an amber suppressor tRNA gene, although a low level of leaky expression of the toxin and cell ablation was seen in the absence of the amber suppressor tRNA (FBrf0054072).