Proteasomes generate peptides that are presented by major histocompatibility complex (MHC) I molecules to other cells of the immune system. Proteolysis is conducted by 20S proteasomes, complexes of 28 subunits arranged as a cylinder in 4 heteroheptameric rings: alpha-1 to -7, beta-1 to -7, beta-1 to -7, and alpha-1 to -7. The catalytic subunits are beta-1 (PSMB6; MIM 600307), beta-2 (PSMB7; MIM 604030), and beta-5 (PSMB5; MIM 600306). Three additional subunits, beta-1i (PSMB9; MIM 177045), beta-2i (PSMB10; MIM 176847), and beta-5i (PSMB8; MIM 177046), are induced by gamma-interferon (IFNG; MIM 147570) and are preferentially incorporated into proteasomes to make immunoproteasomes. PSMB11, or beta-5t, is a catalytic subunit expressed exclusively in cortical thymic epithelial cells (Murata et al., 2007 [PubMed 17540904]).