code: @Override
  	public String getSeqResSequence(){
  
  		StringBuilder str = new StringBuilder();
  		for (Group g : seqResGroups) {
  			ChemComp cc = g.getChemComp();
  			if ( cc == null) {
  				logger.warn("Could not load ChemComp for group: ", g);
  				str.append(StructureTools.UNKNOWN_GROUP_LABEL);
  			} else if ( PolymerType.PROTEIN_ONLY.contains(cc.getPolymerType()) ||
  					PolymerType.POLYNUCLEOTIDE_ONLY.contains(cc.getPolymerType())){
  				// an amino acid residue.. use for alignment
  				String oneLetter= ChemCompGroupFactory.getOneLetterCode(cc);
  				// AB oneLetter.length() should be one. e.g. in 1EMA it is 3 and this makes mapping residue to sequence impossible.
  				if ( oneLetter == null || oneLetter.isEmpty() || oneLetter.equals("?")) {
  					oneLetter = Character.toString(StructureTools.UNKNOWN_GROUP_LABEL);
  				}
  				str.append(oneLetter);
  			} else {
  				str.append(StructureTools.UNKNOWN_GROUP_LABEL);
  			}
  		}
  		return str.toString();
  	}
docs: {@inheritDoc}