Argyrin B is a naturally occurring cyclic peptide that acts as a reversible non-competitive inhibitor of the immunoproteasome. It exhibits higher inhibitory selectivity for the β5i and β1i sites of the immunoproteasome compared to the β5c and β1c sites of the constitutive proteasome, being nearly 20 times more selective for β1i than for its homolog β1c. Additionally, Argyrin B possesses antibacterial properties.