An autosomal recessive variant of the SACS gene causing cerebellar ataxia has been identified in Pyrenean mountain dogs. The first clinical signs appear at around 4 months of age and include clumsiness, uncoordinated behavior and difficulty walking on slippery surfaces. Affected puppies are reluctant to go up or down stairs and often look for fences and walls to lean against when walking. If the opportunity arises, they lie down (e.g. when eating). The symptoms progress slowly over time and lead to neuromuscular weakness as the predominant symptom. Physical activity does not lead to a worsening of the symptoms. Blood tests revealed no abnormalities. MRI and pathological examinations showed a striking reduction in the size of the cerebellum. Most of the dogs were euthanised at the age of 4-7 years due to the symptoms.