Editor—Spinocerebellar ataxia type 6 (SCA6) is a dominantly inherited form of spinocerebellar ataxia, which usually manifests as a syndrome of progressive pure cerebellar ataxia with a late onset, and is caused by a small expansion of CAG repeats in the coding region of the α_lA subunit of the P/Q type voltage dependent calcium channel gene (CACNA1A).1 In the original study, normal alleles were shown to have four to 16 CAG repeats, while expanded alleles with 21-27 CAG repeats were specifically associated with the clinical disorder. Subsequent studies have shown that pathogenic expansion ranges from 21-33 CAG repeats and that its size is inversely correlated with the age of onset,2-9 whereas the majority of normal alleles have 4-18 repeats and a few carriers possess 19-20 repeats.2 The frequency of relatively larger alleles in the normal population correlates with the prevalence of various forms of dominant SCA caused by tandem repeat expansion10 and the intermediate allele of SCA6 itself can potentially harbour de novo mutations.11 In all of these studies, an allele with 21 CAG repeats was considered to be the smallest expansion generating the clinical disorder.1-3 6-9 However, it is still uncertain whether 20 repeats can also cause clinical manifestations. We describe here two sibs with SCA6, who were both homozygous for 20 repeat alleles and presented with late onset pure cerebellar …