BACKGROUND: The cooccurrence of intermediate (40-49 CAG/CAA) TBP repeat expansions with STUB1 variants questions the pathogenicity of monoallelic STUB1 variants in cerebellar ataxia. OBJECTIVE: The objective of this study was to describe the phenotypic spectrum of heterozygous STUB1 variants with or without intermediate TBP repeat expansions. METHODS: We determined the presence of TBP repeat expansions and STUB1 variants in six families with cerebellar ataxia. RESULTS: Cooccurrence of both genotypes in one family resulted in cerebellar ataxia, involving cognitive and extrapyramidal complications. Variable degrees of cerebellar ataxia and cognitive impairment were found in four families carrying a heterozygous STUB1 variant and normal TBP alleles. Finally, we report one patient with a mild late-onset cerebellar ataxia carrying an intermediate expanded TBP allele without the presence of a STUB1 variant. CONCLUSIONS: Heterozygous STUB1 variants are associated with a milder phenotype and reduced penetrance compared with the cosegregation with intermediate TBP alleles, which causes a fully penetrant complicated form of cerebellar ataxia. © 2025 International Parkinson and Movement Disorder Society.