ATXN1 and Parkinson disease: For ATXN1 gene CAG repeats size, the best logistic model (P = 0.001, AIC = 1666.1) included the quadratic term of short allele (P = 0.024), the long allele (0.021) and the interaction between both alleles (P = 0.017) as predictors for the Parkinson’s disease risk.