The differential diagnosis in an adolescent with paroxysmal dyskinesias and ataxia includes a broad range of conditions, such as autosomal recessive cerebellar ataxias (ARCA, e.g., SETX, SYNE1 mutations), ataxia telangiectasia, organic acidemias, and juvenile-onset mitochondrial epileptic syndromes. The gene discussed is SYNE1; the disease is inborn organic aciduria.