Hello all, I am refining three protein-DNA complexes each with a different modified nucleotide-monophosphate residue at the 3'-terminus of its DNA strand. Two of these worked fine after sending the pdb file through ready-set. With the third structure the modified nucleoside-monophosphate residue is not recognized as linked to the previous nucleotide-monophospahe--no phosphodiester bond formation. The only difference between the two that "worked" and the one that didn't is the nature of the deoxyribose sugar modification. The first two have substituents at the 4'-carbon of the deoxyribose sugar, the third has the 4'-oxygen of the deoxyribose sugar substituted with a -CH2- making it a cylcopentyl pseudo-sugar. The bond lengths and angles are reasonable in the starting model, I had no problem doing the initial refinement in CNS after defining topology and parameter files. The output for this in the log file is: Chain: "P" Number of atoms: 439 Number of conformers: 1 Conformer: "" Number of residues, atoms: 21, 439 Unusual residues: {'CAR': 1} Classifications: {'undetermined': 1, 'DNA': 20} Modifications used: {'5*END': 1} Link IDs: {None: 1, 'rna3p': 19} Not linked: pdbres="CYT P 821 " segid="P " pdbres="CAR P 822 " segid="P " Thanks, Steve