PDB FORMAT Version 3.0.1
Main Index

Heterogen Section

The heterogen section of a PDB file contains the complete description of non-standard residues in the entry. Changes in the detailed chemical descriptions of non-polymer chemical components are described in the PDB Chemical Components dictionary,


There are no character/column format changes in the records in this section; however, the definition of a PDB HET group is revised owing to the change in nomenclature for the standard deoxyribonucleotides as described in the following section.


HET records are used to describe non-standard residues, such as prosthetic groups, inhibitors, solvent molecules, and ions for which coordinates are supplied. Groups are considered HET if they are not part of a biological polymer described in SEQRES and considered to be a molecule bound to the polymer, or they are a chemical species that constitutes part of a biological polymer that is not one of the following:

  • not one of the standard amino acids, and
  • not one of the ribonucleic acids (C, G, A, T, U, and I), and
  • not one of the deoxyribonucleic acids (DC, DG, DA, DT, DU and DI)
  • not an unknown amino acid or nucleic acid where UNK is used to indicate the unknown residue name.

HET records also describe chemical components for which the chemical identity is unknown, in which case the group is assigned the hetID UNL (Unknown Ligand).

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