Sialuria, Finnish type, also known as Salla disease, is a lysosomal storage disorder caused by defective transport of lysosomal degradation product, free sialic acid, across the lysosomal membrane. The proton-driven transport is mediated by sialin, a transmembrane protein with 12 transmembrane domains coded by a gene SLCL17A5 at chromosome 6q13-14. Lysosomal accumulation of free sialic acid takes place in a variety of cell types and tissues but functional consequences come only from central nervous system presenting slowly progressive mental retardation associated with ataxia and some other neurological manifestations. Affected infants present first signs of the disease already at 3 to 9 months of age as muscular hypotonia, truncal and limb ataxia, often transient nystagmus and delayed motor development. The developmental profile of SD patients is characterized by slowly progressive general handicap with motor performance more severely and earlier affected than cognitive skills. All patients are severely mentally retarded from the third decade on. Life span of affected patients is close to normal. MRI studies of the central nervous system have documented a fairly common dysmyelination pattern and hypoplasia of corpus callosum. Genotype-phenotype correlation exists in SD; 95% of the Finnish patients carry the p.Arg39Cys founder mutation and present with "classical" phenotype whereas cases with compound heterozygote genotypes are more severely affected, have epileptic convulsions and shorter life span. The phenotypic spectrum of SLC17A5 mutations also includes ISSD (Infantile sialic acid storage disease, OMIM#269920), a severe disease of newborns and young infants leading to early death. Diagnosis of SD is based on excess of free sialic acid in urine and cultured cells or on mutation detection in the SLC17A5 gene. A gene test also allows fetal diagnosis and carrier identification.