The immunoproteasome is an alternative form of the constitutive proteasome that is a key component of the proteolytic machinery. When the cells are stimulated by inflammatory stimuli, such as interferon-γ or TNF-α, three subunits of the constitutive proteasome β1, β2, and β5 are replaced with the immunoproteasome catalytic subunits β1i (low-molecular mass protein [LMP] 2), β2i (multicatalytic endopeptidase complex-like [MECL] 1), and β5i (LMP7), respectively. LMP7 is required to produce proinflammatory cytokines, such as TNF-α and IL-6, and to progress experimental arthritis and colitis. LMP7 is predominantly expressed in immune and inflammatory cells. Immunoproteasome has diverse functions such as regulation of key processes in maintaining cellular homeostasis and in responding to stress other than antigen presentation.