In the acidic environment of the lysosome, hyaluronidase 1 (HYAL1) is able to hydrolyse large 20kDa HA fragments (approximately 50 disaccharide units) to 800 Da fragments (2 disaccharide units). 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).