Proteasome subunit beta type-9 as known as 20S proteasome subunit beta-1i is a protein that in humans is encoded by the PSMB9 gene. This protein is one of the 17 essential subunits (alpha subunits 1-7, constitutive beta subunits 1-7, and inducible subunits including beta1i, beta2i, beta5i) that contributes to the complete assembly of 20S proteasome complex. In particular, proteasome subunit beta type-5, along with other beta subunits, assemble into two heptameric rings and subsequently a proteolytic chamber for substrate degradation. This protein contains "Trypsin-like" activity and is capable of cleaving after basic residues of peptide. The eukaryotic proteasome recognized degradable proteins, including damaged proteins for protein quality control purpose or key regulatory protein components for dynamic biological processes. The constitutive subunit beta1, beta2, and beta 5 (systematic nomenclature) can be replaced by their inducible counterparts beta1i, 2i, and 5i when cells are under the treatment of interferon-γ. The resulting proteasome complex becomes the so-called immunoproteasome. An essential function of the modified proteasome complex, the immunoproteasome, is the processing of numerous MHC class-I restricted T cell epitopes.