An RNA strand is formed by nucleotide sequences.
Nucleotide sequences of RNA are denoted by A, C, G, and U.
The residue sequence number in PDB files.
Chain identifier. It corresponds to the sequence number. More than one chain ID (e.g., A,B) means that corresponding nucleotides (e.g, 12-15,22-30) come from chain A and chain B, respectively.
Nucleotide residues are represented by sequence, number, and chain ID.