In eukaryotic cells, selective breakdown of cellular proteins is ensured by their ubiquitination and subsequent degradation by the 26S Proteasome. The 26S Proteasome is a protease complex that selectively breaks down proteins that have been modified by polyubiquitin chains. It is made up of two multisubunit complexes: the 20S Proteasome chamber, which serves as the proteolytic core of the complex and two 19S regulatory particles which recognize and unfold ubiquitinated proteins. PSMD12 (proteasome (prosome, macropain) 26S subunit, non-ATPase, 12), also known as p55 or Rpn5, is a 456 amino acid protein belonging to the proteasome subunit p55 family. PSMD12 acts as a regulatory subunit of the 26S proteasome and is a component of the PA700 complex. PSMD12 contains one PCI domain.