PRotein Ontology (PRO) Release 63.0 (10-May-2021) There are 230455 PRO terms in the Protein Ontology. Those representing individual proteins are mapped to 139626 UniProtKB sequences. 520 terms are in the 'family' category. 23470 terms are in the 'gene' category. 23 terms are in the 'seqgroup' category. 8067 terms are in the 'sequence' category. 4976 terms are in the 'modification' category. 221 terms are in the 'complex' category. 38 terms are in the 'organism-family' category. 100075 terms are in the 'organism-gene' category. 202 terms are in the 'organism-seqgroup' category. 71273 terms are in the 'organism-sequence' category. 15632 terms are in the 'organism-modification' category. 464 terms are in the 'organism-complex' category. 24 terms are in the 'union' category. 72 terms are in the 'external' category. 2585 terms have some kind of annotation, codifying the information from 1656 papers. 4442 connections to GO (1721 PRO terms). 616 connections to Pfam (369 PRO terms). 27 connections to SO (24 PRO terms). 610 annotations of a phenotype (585 PRO terms). The ontology includes a subset of terms from other ontologies and resources that are used for logical definitions. _Current changes_ (None) _Forthcoming changes_ 1) A new synonym type "PRO-org-label" will be added to appropriate terms. Currently the PRO-short-label synonyms use an orthology-based mechanism to identify related terms across species. The new synonym type will identify related terms based on the gene name in the given organism. 2) The use of both EXACT and NARROW PRO-proteoform synonyms will be systematically implemented. 3) We will switch the only_in_taxon relation to in_taxon, in anticipation of the likely deprecation of the former in favor of the latter.