The function of PSMB9 (proteasome 20S subunit beta 9, Ensembl gene identifier ENSG00000240065) is as follows. The proteasome is a multicatalytic proteinase complex which is characterized by its ability to cleave peptides with Arg, Phe, Tyr, Leu, and Glu adjacent to the leaving group at neutral or slightly basic pH (PubMed:33727065, PubMed:34819510). The proteasome has an ATP- dependent proteolytic activity. This subunit is involved in antigen processing to generate class I binding peptides. Replacement of PSMB6 by PSMB9 increases the capacity of the immunoproteasome to cleave model peptides after hydrophobic and basic residues. {ECO:0000269|PubMed:33727065, ECO:0000269|PubMed:34819510, ECO:0000269|PubMed:8163024}.