Truman Xu

4 out of 6 entries in the dictionary exist in structures in PDB (a total of 90 structures), and you would fail when you read the sequence from these structures...

But what is the harm in including those in the extended 3to1 dict?

Sorry, I have trouble understanding this. Isn't the `protein_letters_3to1_extended` supposed to be everything including the residue names defined by IUPAC? The current one does not include the reverse mapping of...

For example, ```python from Bio.PDB.Polypeptide import protein_letters_3to1_extended protein_letters_3to1_extended['SEC'] #SELENOCYSTEINE (U) ``` This would raise a KeyError

Moreover, the 20 canonical residue names are indeed in the `protein_letters_3to1_extended` as expected ```python from Bio.Data.PDBData import protein_letters_3to1 from Bio.Data.PDBData import protein_letters_3to1_extended canonical_resnames = set(protein_letters_3to1.keys()) extended_resnames = set(protein_letters_3to1_extended.keys()) canonical_resnames.issubset(extended_resnames) ```...

Got it! Thank you for the explanations!