Salla disease (SD) and infantile sialic acid storage disorder (ISSD) are rare autosomal recessive lysosomal storage diseases caused by mutations in SLC17A5 gene which codes for the protein sialin. Sialin is lysosomal membrane transporter that exports free sialic acid from lysosomes and deficient of sialin results in excessive lysosomal storage of free sialic acid. ISSD has a severe phenotype with infantile onset, while the SD has a milder phenotype with later onset. Both disorders cause developmental delay, and ISSD is generally fatal in early childhood. The differential diagnosis of free sialic acid storage also includes French type sialuria, a disorder due to mutations in the UDP-GlcNAc 2-epimerase (GNE), the key enzyme for the biosynthesis of sialic acid.
