SCA6 is caused by CAG expansion in the alpha 1A voltage-dependent calcium channel subunit gene. The authors studied an Italian family in which one patient carried a fully expanded SCA6 allele with 26-CAG repeats, whereas the other affected family member was homozygous for an intermediate-size allele of 19-CAG repeats. Three family members, heterozygous for the intermediate allele, were clinically unaffected. The findings demonstrate a dose-dependent pathogenic effect of an intermediate CAG expansion in the SCA6 gene.