NEU1 and sialidosis: Two genetically distinct lysosomal storage diseases are associated with NEU1 deficiency: sialidosis (OMIM 256550), which results from structural mutations at the NEU1 locus on chromosome 6p21[55], and galactosialidosis (OMIM 256540), which is caused by a primary defect of PPCA, leading to secondary and combined deficiencies of NEU1 and β-GAL[56,57].