Mucopolysaccharidosis IIIB, an autosomal recessive lysosomal storage disorder of heparan sulfate caused by mutations in the alpha-N-acetylglucosaminidase (NAGLU) gene, was recently discovered in cattle. Clinical signs include progressive ataxia, stumbling gait, swaying and difficulty in balance and walking. These clinical signs are usually first observed at approximately 2 years of age and then develop progressively over the lifespan of the animals. Affected bulls were found to be homozygous for the missense mutation E452K (c.1354G > A). The availability of mutational analysis permits screening for the NAGLU mutation to eradicate this mutation from the cattle breeding population.