Immunoproteasome activator 1 (compound A) is a selective immunoproteasome activator that increases the presentation of individual MHC-I –bound peptides by over 100-fold. Immunoproteasome activator 1 binds to the proteasome structural subunit PSMA1 and promotes the association of the proteasome activator PA28α/β (PSME1/PSME2) with immunoproteasomes.