Co-chaperone which acts as a regulator of the Hsp70 chaperone machinery and may be involved in the processing of other ataxia-linked proteins. Defects in SACS are the cause of spastic ataxia Charlevoix-Saguenay type (SACS). It is a neurodegenerative disease characterized by early-onset cerebellar ataxia, spasticity, retinal hypermyelination, pyramidal signs, and both axonal and demyelinating neuropathy with loss of sensory nerve conduction and reduced motor conduction velocities. Other features include dysarthria, distal muscle wasting, nystagmus, defect in conjugate pursuit ocular movements, retinal striation (from prominent retinal nerves) obscuring the retinal blood vessels in places, and the frequent presence of mitral valve prolapse. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.