We observed a marked decrease in CAST levels in HdhQ111 striatal cells, HdhQ7 striatal cells exposed to 3-nitropropionic acid (3-NP; a mitochondrial toxin that causes HD-like symptoms in rodents and primates38), and HEK293 cells overexpressing mtHtt carrying either 73Q or 145Q when compared with the control groups (Supplementary Fig. 4b–d). Here, CAST is linked to Huntington disease.