Notably, recent genome-wide association studies (GWASs) have successfully identified genetic modifiers that influence the AOO in Huntington’s disease by analysing thousands of affected individuals.7 Among these modifiers are several genes related to DNA mismatch repair, including FAN1, LIG1, MLH1, MSH3, PMS1 and PMS2,7 which are thought to impact the somatic expansion of the CAG repeat in vulnerable neural cell types. Here, FAN1 is linked to juvenile Huntington disease.