ATXN1 and Parkinson disease: We generated models using the CAG repeat sizes HTT, ATXN1 and ATXN2, the quadratic term for each allele, the product term of the two alleles and finally the presence of the IAs as explanatory variables.25,26 The quadratic term for each allele allows us to explore the non-linear relationships between the CAG repeat size of the alleles and Parkinson’s disease risk or cancer risk.