26S proteasome non-ATPase regulatory subunit 2, also as known as 26S Proteasome Regulatory Subunit Rpn1 (systematic nomenclature), is an enzyme that in humans is encoded by the PSMD2 gene. As the degradation machinery that is responsible for ~70% of intracellular proteolysis, proteasome complex (26S proteasome) plays a critical roles in maintaining the homeostasis of cellular proteome. Accordingly, misfolded proteins and damaged protein need to be continuously removed to recycle amino acids for new synthesis; in parallel, some key regulatory proteins fulfill their biological functions via selective degradation; furthermore, proteins are digested into peptides for MHC class I antigen presentation.