Hyaluronidase 1 (HYAL1) hydrolyses 1-4 linkages between GalNAc and D-glucuronate residues in chondroitin (or dermatan). It also hydrolyses this linkage in hyaluronate, another glycosaminoglycan (GAG) composed of repeating disaccharide units but the only one which is non-sulfated. Defects in HYAL1 (MIM:607071) cause mucopolysaccharidosis type IX (MPS IX, Natowicz syndrome, Hyaluronidase deficiency, MIM:601492), a rare lysosomal storage disease. Triggs-Raine et al. identified a patient with two mutations in HYAL1 alleles, a nonconservative amino acid substitution (Glu268Lys) and a complex intragenic rearrangement (1361del37ins14) that results in a premature termination codon (Triggs-Raine et al. 1999).