A congenital disorder of glycosylation I that is characterized by a severe encephalopathy with axial hypotonia, abnormal eye movement, pronounced psychomotor retardation, peripheral neuropathy, cerebellar hypoplasia, and retinitis pigmentosa and has_material_basis_in homozygous or compound heterozygous mutation in the gene encoding phosphomannomutase-2 on chromosome 16p13.