The N188S mutation in Gaucher disease is associated with myoclonus epilepsy. We performed genetic analysis on a patient with progressive myoclonus epilepsy, who had received antiepileptic drugs for over 10 years. We detected N188S/G199D on the gene encoding glucocerebrosidase. Mutant proteins carrying each mutation were expressed in COS-1 cells (a commonly used cell line which derives from kidney cells of the African green monkey). Measurements of enzymatic activity and Western blotting analysis were performed. When residual activities were measured, glucocerebrosidase with the N188S mutation exhibited 50% activity of the wild type, and with G199D, 7.4%. Neither mutation influenced the stability of the enzyme protein. These data suggested a diagnosis of Gaucher disease for this patient, and indicated that G199D is a novel mutation.