Notably, the clinical severity of Huntington’s disease does not always correlate with neuropathological stage.39 In fact, the age of onset of motor symptoms is inversely correlated with the length of the CAG triplet repeat,40 and the triplet repeat is the only main factor, as is the alteration of given genetic loci, such as FAN1, MSH3 or MLH1.41 Therefore, further research is needed to characterize the relationships among clinical severity, pathological grade, HTT deposits and CAG repeats. This evidence concerns the gene FAN1 and juvenile Huntington disease.