Protein Modification

PSI-MOD is an ontology consisting of terms that describe protein chemical modifications, logically linked by an is_a relationship in such a way as to form a direct acyclic graph (DAG). The PSI-MOD ontology has more than 45 top-level nodes, and provides alternative hierarchical paths for classifying protein modifications either by the molecular structure of the modification, or by the amino acid residue that is modified.

Ontology information

Ontology ID: mod
Number of terms: 2098
Last loaded: Mon Jun 20 06:40:53 GMT 2022
Creator: Human Proteome Organization (HUPO) Proteomics Standards Initiative (PSI) (


13:06:2021 12:12


Joshua Klein


Annotation note 05 - In entries with an isotope indicator in the formula, average masses are meaningless and are assigned the value equal to the monoisotopic mass, but rounded to a lower precision; monoisotopic masses are calculated by using the masses for the indicated isotopes and the most common isotopes for other elements without isotope indicators in the formulas.

Annotation note 10 - GNOme is the Glycan Naming and Subsumption Ontology (, an ontology for the support of glycomics. PSI-MOD does not have all possible glycans in its entries, just the ones that are noted to be on proteins and have been requested for addition. GNOme uses GlyTouCan ( to provide stable accessions for glycans described at varyious degrees of characterization, including compositions (no linkage) and topologies (no carbon bond positions or anomeric configurations).

PSI-MOD version: 1.031.6

Annotation note 09 - The Open Mass Spectrometry Search Algorithm, OMSSA, enumerated list of modifications are being incorporated. The string values are synonyms with the synonymtypedef "OMSSA-label", and their integer values (which are supposed to be stable) are definition cross-references.

Annotation note 06 - For cross-link modifications, the number following "Cross-link" in the comment record indicates the number of amino acid residues that appear in the origin record, used to check the difference formula and masses. This usage differs from RESID, where the cross-link number indicates the maximum number of peptide chains that can be cross-linked.

Annotation note 03 - When an entry in the Unimod database is annotated as a general modification, and one or more instance sites are listed, then the PSI-MOD definition for each different site instance carries the Unimod cross-reference followed by a hash symbol and an amino acid code, "N-term" or "C-term".

Annotation note 04 - The elemental formulas are in strict alphabetical order, not in CAS ("C" and "H" first) order. Isotope numbers are in parentheses before the element symbol, and an element should not occur in a formula both with and without an isotope number. In difference formulas, counts can be zero or negative.

RESID release: 75.00

Annotation note 07 - The synonym cross-reference "MOD:old name" has been replaced by "MOD:alternate name".

Annotation note 08 - The DeltaMass listings for free amino acids have been removed. Most Unimod entries that have not been "approved" have by general agreement not been incorporated unless there has been a request for a specific term by a PRIDE submitter.

Annotation note 01 - "[PSI-MOD:ref]" has been replaced by PubMed:18688235.

Annotation note 02 - When an entry in the RESID Database is annotated with different sources because the same modification can arise from different encoded amino acids, then the PSI-MOD definition for each different source instance carries the RESID cross-reference followed by a hash symbol "#" and a 3 or 4 character label. When an entry in the RESID Database is annotated as a general modification with the same enzymatic activity producing different chemical structures depending on natural variation in the nonproteinaceous substrate, on secondary modifications that do not change the nature of the primary modification, or on a combination of a primary and one or more secondary modifications on the same residue, then the PSI-MOD definition for each different instance carries the RESID cross-reference followed by the special tag "#var".

ISO-8601 date: 2022-06-13 12:12Z