SLC17A5 encodes a lysosomal sialic acid transporter, Sialin (AST, membrane glycoprotein HP59) (Verheijen et al. 1999, Fu et al. 2001). SLC17A5 exports sialic acid (N-acetylneuraminic acid, Neu5Ac) which is derived from the degradation of glycoconjugates. This export is dependent on the proton electrochemical gradient across the lysosomal membrane. SLC17A5 is present in the pathological tumor vasculature of the lung, breast, colon, and ovary, but not in the normal vasculature, suggesting that the protein may be critical to pathological angiogenesis. Sialin is not expressed in a variety of normal tissues, but is significantly expressed in human fetal lung. Defects in SLC17A5 cause Salla disease (SD) and infantile sialic acid storage disorder (ISSD aka N-acetylneuraminic acid storage disease, NSD). These belong to the sialic acid storage disease (SASD) group and are autosomal recessive neurodegenerative disorders characterised by hypotonia, cerebellar ataxia and mental retardation in very young infants (Verheijen et al. 1999, Aula et al. 2000).