Hap1 was associated with reduced somatic expansion in DM1 (P = 0.032) and slower progression in Huntington’s disease (P = 0.020), whereas Hap2 was associated with reduced somatic expansion (P = 0.021) and delayed onset (P = 4.03 × 10−5) in both Huntington’s disease and DM1, and with slower progression (P = 1.64 × 10−5) and reduced expression of MSH3 (P = 0.024) and DHFR (P = 1.12 × 10−3) in Huntington’s disease (Supplementary Table 9). Here, HAP1 is linked to myotonic dystrophy type 1.