The present invention relates to a composition comprising a diphtheria toxin (DT) variant, said DT variant comprises an amino acid sequence as recited in SEQ ID NO: 4 with one or more amino acid substitutions or deletions or combinations thereof, wherein at least one amino acid substitution or deletion is made within the regions selected from the group consisting of residues 7-9, 29-31 and 290-292 of SEQ ID NO: 4, and wherein said DT variant has cytotoxicity comparable to that of a DT molecule having a sequence of SEQ ID NO: 4.
