FAN1 and Huntington disease: An intense focus on somatic CAG expansions as a therapeutic target arose in 2015 from a series of screens for human modifiers of disease age-of-onset (AOO) or progression in HD individuals, which revealed the DNA repair proteins MSH3, FAN1, PMS1, PMS2, MLH1, and LIG1 as disease modifiers2,3,35–37.