HTT-D3 is a potent and orally active huntingtin (HTT) splicing modulator. HTT-D3 acts by promoting the inclusion of a pseudoexon containing a premature termination codon (stop-codon psiExon), leading to HTT mRNA degradation and reduction of HTT levels. HTT-D3 reduces p-glycoprotein (P-gp) efflux, and can be uesd for Huntington's disease research.