No output when trying to obtain protein sequence from PDB file I am trying to obtain the sequence from PDB file, but no output is given when I run the following code :  

from Bio import SeqIO

with open("1a9l.pdb", "r") as handle:
    for record in SeqIO.parse(handle, "pdb-atom"):
        print(record.seq)
That PDB entry is an RNA molecule, so there is no protein sequence to extract.

If it extracted the RNA sequence by default here you can see there could be an ambiguity as to whether it was an RNA sequence or a protein sequence, as the alphabets share characters.