ATXN2 and Parkinson disease: The best logistic model for prediction of Parkinson’s disease risk in reference to the ATXN2 CAG repeats (P = 1.68–06; AIC = 2037) included as predictors the quadratic term of the short allele (P = 2.72e−04), the long allele (P = 0.002) and the interaction between both alleles (P = 0.002).