The PSI-MS (Mass Spectrometry) CV was initially developed to serve the needs of the PSI mass spectrometry-related data formats, most of which make extensive use of CV terms to precisely describe metadata and provide extensibility as technologies advance . The CV was first developed to support the now-deprecated mzData format and then for its replacement mzML, and has since been adapted for numerous additional formats that will be further described below. It contains many terms for specific instrument models, software packages, and other metadata concepts and continues to grow as more such terms become available. This CV is readily accessible at and is typically updated at least monthly. The contents of the CV is also browsable via the Ontologies Lookup Service () and the NCBO BioPortal (). PSI-MOD is an ontology of modified amino acid residues. The terms focus on the end products of modifications rather than modifications themselves. Thus, instead of terms for phosphorylation and oxidation, there are terms such as Ophospho-L-serine and L-methionine sulfoxide. The concepts are organized in several hierarchies by amino acid and by modification type. Although the ontology suffered from a period of inactivity several years ago, volunteers have now stepped forward to maintain it actively. The ontology is maintained on GitHub () or is readily available through the Ontology Lookup Service (). Complementary CVs, maintained by other organizations, which contain information on protein modifications include RESID, PTMList, Unimod, and XLMOD. PSI-MOD was originally based on RESID, which is now mostly deprecated and used by very few tools. UniProtKB uses its own PTMList resource (), although PTMList is substantially synchronized with PSI-MOD. The popular Unimod community resource of mass modifications () instead focuses on the modifications themselves (with terms like "Phospho" and "Oxidation") and is a flat CV with no relationship structure.