PRPS1 and Down syndrome: Brown-Vialetto-Van Laere syndrome associated with SLC52A2 mutations is inherited as an autosomal recessive condition (Fig. 6), which helps to differentiate it from other optico-acoustic neuropathies including autosomal dominantly inherited OPA1 or MFN2 mutations, X-linked PRPS1 mutations and mitochondrially inherited neuropathy caused by mitochondrial DNA mutations (such as Leber’s hereditary optic neuropathy or the syndrome of neuropathy, ataxia and retinitis pigmentosa known as ‘NARP’).