Linear regression analysis of SCA3. A total of 308 SCA3 patients with expanded CAG repeats in the ataxin-3 gene was analyzed. Inspection of logarithmically transformed mean age-at-onset versus CAG size plots (A) identified 64, 82 and 83 CAG repeats as relative outliers. However, in contrast to the other polyQ diseases, natural log-transformed age-of-onset versus CAG size was the best fit to a negative quadratic function (B). Clearly, this indicates that the nucleated growth polymerization model is unsuitable for SCA3.