Here we report a 7-year-old Caucasian boy with a pathogenic missense mutation in STXBP1 in whom mitochondrial dysfunction had been suspected based on having severe neurologic dysfunction (intractable mixed epilepsy, ataxia, tremor, appendicular hypertonia, clonus, dysmetria, speech regression, and delayed myelination) and ophthalmologic involvement (variably interpreted retinal pigment changes, abnormal OCT and ERG, and good visual function) with mild lactic acidemia at the time of intercurrent illness. The gene discussed is STXBP1; the disease is cerebellar ataxia.