Polymer sequence in uppercase letters. For DNA, usually A,C,G,T letters representing the nucleosides of adenine, cytosine, guanine and thymine, respectively; for RNA, usually A, C, U, G; for protein, usually the letters corresponding to the 20 letter IUPAC amino acid code.
Source:http://www.biopax.org/release/biopax-level3.owl#sequence