Trinucleotide expansions in the gene for the TATA-binding protein (TBP) have recently been described in cerebellar ataxia associated with dementia, pyramidal tract and basal ganglia symptoms. Expansions above 45 repeat units are commonly considered pathological, causing SCA17. Here, we present a German kindred with four siblings affected by cerebellar ataxia, chorea and dementia. Molecular genetic analysis yielded an expanded SCA17 allele coding for 48 glutamine residues that was transmitted from the mother to all of her six children. Apparently, the expanded allele does not cosegregate with the disease phenotype since the mother and two of the siblings do not show any clinical abnormality. This appears to be the first description of non-penetrance in SCA17.