The SLC17A5 gene, which encodes sialin, has been originally reported as the gene responsible for sialic acid storage diseases (SASD), which are autosomal recessive neurodegenerative disorders, characterized by hypotonia, cerebellar ataxia, and mental retardation. SASD is classified into a severe infantile form (infantile sialic acid storage disease; ISSD) or a slowly progressive adult form (Salla disease, which is prevalent in Finland). The SLC17A5/sialin protein is a lysosomal membrane protein, which is structurally homologous to members of the anion/cation symporters, and transports sialic acid out of lysosomes. It also mediates the transportation of asparate and glutamate in the hippocampal synaptic vesicles and synaptic-like microvesicles. Mouse slc17a5/sialin with a mutant H183R, which is found in ISSD, shows active energy-dependent transportation of asparate and glutamate, but inactive H+/sialic acid cotransportation. In contrast, another mutated slc17a5/sialin with the R39C variant, which is found in patients with Salla disease, retains sialic acid/H+ cotransport activity, but is defective in the uptake of asparate and glutamate. Therefore, severe functional illnesses of the central nervous system in patients with Salla disease are possibly caused by the impaired transport of asparate and glutamate due to the R39C mutation in SLC17A5/sialin.