NEU1 and lysosomal storage disease: The different expression of three enzymes can be explained by the fact that deficiency of each leads to three distinct lysosomal storage disorders (LSDs): galactosialidosis (GS) or PPCA deficiency with a secondary combined deficiency of NEU1 and β-GAL, sialidosis or NEU1 deficiency, and GM1-gangliosidosis (GM1) or β-GAL deficiency.