HTT and Huntington disease: In striatum of an HD mouse model overexpressing relative levels of the exon 1 HTT N-terminal fragment (R6/2 mice), levels of mtDNA were reduced,15 and in STHdhQ7 and STHdhQ111, mouse striatal cells expressing mutant HTT, more mtDNA lesions were observed.16 In leucocytes and fibroblasts from human HTT mutation carriers, mtDNA levels (mtDNA/nuclear DNA ratio) were lower than in controls.17-19