Updated project metadata. In this study we have compared the proteomes from cerebellar synaptosome of early symptomatic Cstb-/- and wild type mice to gain insight into disease onset and progression of cystatin B deficiency. Biallelic loss-of-function mutations in the CSTB gene cause progressive myoclonus epilepsy of Unverricht-Lundborg type (EPM1). In the mouse model of EPM1, CSTB deficiency manifests from one month of age as progressive neurodegeneration, myoclonus and ataxia, which is preceded by neuroinflammation, alterations in GABAergic signaling, and a widespread rearrangement of the mitochondrial proteome in synapses.