Mutant SOD1 is degraded by the proteasome and partial inhibition of proteasome activity leads to the formation of large SOD1‐containing aggregates, which is thought to contribute to neuropathology (Hyun et al, 2003). Recent report indicates that the levels of proteasomal 20S constitutive catalytic subunits are significantly reduced in the spinal cord of SOD1G93A mice at an advanced stage of the disease (Kabashi et al, 2012). Similarly, we observed a decreased chymotrypsin‐like proteasomal activity in NSC‐34 SOD1 G93A cells (P = 0.0087) that was restored by P110 treatment (P = 0.0065; Fig 2E).