CXCR4 and diphtheria: In this context, we have developed self-assembling protein nanoparticles which incorporate the de-immunized catalytic domain of Pseudomonas aeruginosa (PE24) exotoxin A or the diphtheria exotoxin (DITOX) domain from Corynebacterium diphtheriae, that specifically target CXCR4-overexpressing (CXCR4+) cancer cells through the T22 peptide ligand.