HTT and Huntington disease: Since the identification of the first single-chain Fv (scFv) antibody, scFvC4, that specifically binds to the N-terminal region of HTT (Lecerf et al., 2001) and suppresses the formation of mutant HTT (mHTT)-positive aggregates in vivo (Wolfgang et al., 2005; Snyder-Keller et al., 2010), several intrabodies that inhibits mHTT aggregation and ameliorates the behavioral phenotypes of various HD animal models have been identified (Wang et al., 2008; Southwell et al., 2009, 2011; Amaro and Henderson, 2016).