- Subjects with diagnosed Unverricht-Lundborg disease (ULD) ascertained by appropriate genetic testing for a homozygous or compound heterozygous mutation in the CSTB gene- Subjects with moderate to severe myoclonus documented by an Action Myoclonussum score of ≥ 30 (evaluation by investigator)-Subjects currently being or having been treated with clonazepam up to the maximum recommended daily dose of 20 mg or up to their individual optimal dose as assessed by the investigator- Subjects currently being or having been treated with valproate up to the maximum recommended daily dose 60 mg/kg or serum levels of 100 mcg/ml or up to their individual optimal dose as specified by the investigator- Male/female subjects from 16 years onwards. Subjects under 18 years may only be included where legally permitted and ethically accepted