docs: def get_pfam_accession_numbers_from_pdb_chain(self, pdb_id, chain):
          '''Note: an alternative is to use the RCSB API e.g. http://www.rcsb.org/pdb/rest/hmmer?structureId=1cdg.'''
          return self.pdb_chain_to_pfam_mapping.get(pdb_id.lower(), {}).get(chain)