The 20S catalytic core is composed of 4 rings of 28 non-identical subunits; 2 rings are composed of 7 alpha subunits and 2 rings are composed of 7 beta subunits. Immunoproteasomes are modified forms of the constitutive 20S that function in MHC Class I antigen processing. During immunoproteasome assembly, the constitutive catalytic subunits (Delta, Z and X) are replaced by the inducible subunits LMP2, LMP7 and MECL, the former two being encoded by MHC. This protein has been purified from human peripheral blood monocytes, which has been screened and is negative for hepatitis B surface antigen, antibodies to hepatitis C virus, HIV type 1 antigens and antibodies to HIV type 1 and 2.