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Tryptophan repressor with (a) no tryptophan bound (PDB ID: 3wrp), (b) one tryptophan bound and (c) both tryptophans bound (PDB ID: 1wrp). Both (a) and (c) have 
symmetry, and it is speculated that 
symmetry is preserved in (b), as has been observed for symmetric allosteric proteins such as arginine repressor [24]. (Online version in colour.)
