MPS IV, also referred to as Morquio syndrome, presents two subtypes: MPS IVA and MPS IVB, caused by the lack of N-acetylgalactosamine-6-sulfate sulfatase (GALNS; MIM 253,000) and β-galactosidase (GLB1; MIM 253,010) respectively. Here, GALNS is linked to mucopolysaccharidosis type 4A.