I have two PDBs:
PDB ID 4TS0 and PDB ID 3SUY. Both these are RNAs, and both contain the residue CCC. However, when I open the PDB file, I see the line "MODRES 3SUY CCC X 101 C" in case of 3SUY but there is no line containing "MODRES" in 4TS0. I do not understand the logic behind this. Could someone please explain?