NDUFS4 and cerebellar ataxia: Symptoms are highly variable, but usually include psychomotor arrest or regression, hypotonia, dystonia, ataxia, abnormal ocular movements or ophthalmoplegia, lethargy, apnoeic spells and respiratory failure.6, 7, 8, 9 Biochemically, elevated lactate levels in the blood and cerebrospinal fluid are frequently detected.10 To date, NDUFS4 mutations have been described in 22 patients from 18 families, with symptom onset between 5 days and 4 months of life.6