rcsb pdb pdbe pdbj heterogen PSI-MI MI:2017 het The 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 one of the standard amino acids, and - not one of the nucleic acids (C, G, A, T, U, and I), and - not one of the modified versions of nucleic acids (+C, +G, +A, +T, +U, and +I), and - not an unknown amino acid or nucleic acid where UNK is used to indicate the unknown residue name. Het records also describe heterogens for which the chemical identity is unknown, in which case the group is assigned the hetID UNK.