Grant, B.J. et al. (2006) Bioinformatics22, 2695--2696.
For a description of IUPAC one-letter codes see:
http://www.chem.qmul.ac.uk/iupac/AminoAcid/
For a description of PDB residue codes see Appendix 4:
http://msdlocal.ebi.ac.uk/docs/pdb_format/appendix.html