SPR and dystonia 5: Rarely, recessively inherited SPR mutations (leaky or partially penetrant splicing variants), which may result in only a slight reduction of SR activity, can also cause DRD (SR-deficient DRD: the very mild form of SR deficiency17)); most of cases with SR deficiency are known to develop more severe symptoms and signs (motor and speech delay, truncal hypotonia, cognitive impairment, psychiatric and behavioral problems, paroxysmal stiffening, etc.)19, 20).