I'm not sure there are other resources in biomedical research that use UNII so perhaps loading these mappings into mygene.info is somewhat low priority for the moment. But it is possibly higher priority as it relates to to their use in BioThings Explorer (https://github.com/biothings/biothings_explorer/issues/131).
The FDA provides Unique Ingredient Identifiers (UNII), primarily for small molecule compounds but also for the proteins that they target. The complete list of UNIIs for proteins with mappings to UniProt can be found using the OpenFDA API (example query: https://api.fda.gov/other/substance.json?search=codes.code_system:%22UNIPROT%22&limit=10). The UniProt mapping can be found under the
codeskey:I'm not sure there are other resources in biomedical research that use UNII so perhaps loading these mappings into mygene.info is somewhat low priority for the moment. But it is possibly higher priority as it relates to to their use in BioThings Explorer (https://github.com/biothings/biothings_explorer/issues/131).