Residues in the TfR dimer interface were identified by contact analysis in CNS25 of the unliganded TfR structure (1CX8.pdb)26 using a probe radius of 1.4 Å and a distance cutoff of 4 Å. The interface is 70% identical between TfR and TfR2, consistent with the observation of TfR-TfR2 heterodimers. Nonconservative substitutions are underlined and semiconservative substitutions are italicized. Nonconservative substitutions with an asterisk could probably be accommodated in the dimer interface, as determined by inspection of the TfR structure. For example, the substitutions at TfR position 668 and 669 would disrupt a cation-π interaction between an arginine and a phenylalanine and replace it with a salt bridge between a glutamate and an arginine.