PSMD4 [proteasome (prosome, macropain) 26S subunit, non-ATPase, 4] is a non-ATPase subunit of the 19S regulator lid. The 19S regulator and 20S core complexes make up the 26S proteasome which is a multicatalytic proteinase complex with a highly ordered structure. The 20S core is composed of 4 rings of 28 non-identical subunits; 2 rings are composed of 7 alpha subunits and 2 rings are composed of 7 beta subunits. The 19S regulator is composed of a base, which contains 6 ATPase subunits and 2 non-ATPase subunits, and a lid, which contains up to 10 non-ATPase subunits. Proteasomes are distributed throughout eukaryotic cells at a high concentration and cleave peptides in an ATP/ubiquitin-dependent process in a non-lysosomal pathway. An essential function of a modified proteasome, the immunoproteasome, is the processing of class I MHC peptides [taken from NCBI Entrez Gene (Gene ID: 5710)].