C-terminal receptor binding (R) domain - binds to cell surface receptor, permitting the toxin to enter the cell by receptor mediated endocytosis.

1: Refined structure of monomeric diphtheria toxin at 2.3 A resolution. Bennett MJ, Eisenberg D; Protein Sci 1994;3:1464-1475. PMID:7833808

2: Crystal structure of diphtheria toxin bound to nicotinamide adenine dinucleotide. Bell CE, Eisenberg D; Biochemistry 1996;35:1137-1149. PMID:8573568