Canonical sequence of protein or nucleic acid polymer in standard
one-letter codes of amino acids or nucleotides,
corresponding to the sequence in
_entity_poly.pdbx_seq_one_letter_code. Non-standard
amino acids/nucleotides are represented by the codes of
their parents if parent is specified in
_chem_comp.mon_nstd_parent_comp_id, or by letter 'X' if
parent is not specified. Deoxynucleotides are
represented by their canonical one-letter codes of A,
C, G, or T.
For modifications with several parent amino acids,
all corresponding parent amino acid codes will be listed
(ex. chromophores).