Similarly, the assay for N-acetylgalactosamine 6-sulfatase (GALNS), the deficient enzyme in MPS IVA uses 4-methylumbelliferyl-β-d-galactopyranoside-6-sulfate as the substrate for enzymatic cleavage of the C-6 sulfate in the first step, followed by cleavage of the glycosidic linkage in the second step (Figure 5). This evidence concerns the gene GALNS and mucopolysaccharidosis type 4A.