HPRT1 and Huntington disease: There is strong evidence that the polyQ expansion induces a gain of function since insertion of an expanded CAG sequence in the hypoxanthine phosphoribosyltransferase (HPRT) gene, an HD-unrelated gene which is not involved in any polyQ disorders, induced late-onset neurodegeneration and premature death in a mouse model similar to transgenic HD mouse models [7].