A series of DNA repair genes including MLH1, MSH3, FAN1, and PMS2 have been implicated in the repeat instability observed in different repeat disorders (Bettencourt et al., 2016; Genetic Modifiers of Huntington's Disease Consortium, 2015; Moss et al., 2017; Tome et al., 2013). Here, FAN1 is linked to juvenile Huntington disease.