HTT and Huntington disease: The recently introduced genetic editing techniques are also being investigated for HD gene therapy; for example, CRISPR/Cas9-induced double-strand breaks could cause CAG repeat contraction in the HTT locus [34], and an RNA-targeting CRISPR–Cas13d system was shown to mitigate HD-related deficits in a mouse model of HD by improving motor coordination, inhibiting striatal atrophy, and decreasing mutant HTT protein aggregates [32].