Sialidases 1-4 (NEU1-4, neuraminidases, receptor-destroying enzymes, RDEs) hydrolyse sialic acids (N-acetylneuraminic acid, Neu5Ac) to produce asialo compounds, a step in the degradation process of glycoproteins and gangliosides and are expressed in a variety of cellular locations. NEU4 is an extrinsic membrane protein associated with lysosomes, mitochondria and endoplasmic reticulum. It has broad sialidase activity against glycoconjugates with alpha2,3-, alpha2,6- or alpha2,8-linkages (Bigi et al. 2010, Monti et al. 2004, Seyrantepe et al. 2004). NEU1 (lysosomal sialidase) hydrolyses Neu5Ac from glycoconjugates with alpha2,3-, alpha2,6- or alpha2,8-linked terminal sialated residues in the lysosomal lumen. NEU1 is active in a multienzyme complex comprising cathepsin A protective protein (CTSA) and beta-galactosidase (Bonten et al. 1996, Rudenko et al. 1995). Defects in NEU1 cause Sialidosis (MIM:256550), a lysosomal storage disorder manifesting as type I (late-onset) or type II (earlier-onset) (Bonten et al. 1996). CTSA is thought to exert a protective function necessary for stability and activity of these enzymes (Galjart et al. 1988). Defects in CTSA are the cause of galactosialidosis (GSL; MIM:256540) (Zhou et al. 1991).