Hap2, the MSH3 haplotype most significantly linked with reduced somatic expansion and delayed onset in Huntington’s disease and DM1, and with slower progression in Huntington’s disease, contains the 3a allele, along with alternative alleles of non-coding variants rs151182735, rs10168 and rs2250063, which are in complete linkage disequilibrium with it, and rs1677658. This evidence concerns the gene HAP1 and juvenile Huntington disease.