To determine whether age-at-onset in HD is modified by the CAG repeats of other polyglutamine disease-causing genes, we determined directly the length of CAG repeats in ATN1 (DRPLA), ATXN1 (SCA1), ATXN2 (SCA2), ATXN3 (SCA3), CACNA1A (SCA6) and TBP (SCA17) in the HD individuals who participated in both the COHORT study (https://clinicaltrials.gov/ct2/show/NCT00313495) and our recent HD modifier GWA analysis.15 The CAG repeat sizes of the polyglutamine disease-causing genes showed distinct distribution patterns. Here, CACNA1A is linked to Huntington disease.