This diphtheria toxin homolog is a nontoxic cross-reacting material (CRM) produced by a strain of corynephage beta that had undergone N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis. This treatment induces guanine-adenine transitions in the DNA; the differences from the wild-type sequence at positions 104, 187, 222, 456 and 532 can be explained by such mutations. Either of the first 2 mutations will completely abolish all ADP-ribosylating activity of fragment A. The last 2 mutations may be responsible for the reduced receptor-binding capacity of this protein. In wild-type active diphtheria toxin, fragment A catalyzes the ADP-ribosylation of elongation factor 2 and blocks protein synthesis in eukaryotic cells; fragment B binds to the receptor protein and is necessary for the movement of fragment A into the cell.