The effect of a CLCN1 frameshift in the acetazolamide-responsive patient (Case-14) with severe ataxia, nausea, vomiting, and nystagmus at the age of 27, raised the possibility of phenotypic variability within the same genotype p.Gly945ArgfsX39 mutation. The gene discussed is CLCN1; the disease is pathologic nystagmus.