A) Graphical experimental overview where 2 month old BACHD animals received a unilateral ICV injection of either PBS or ascending doses HTT ASO (15–150 μg). Four weeks after ICV injections, treated animals were harvested and mHTT levels in different brain regions were quantified by immunoblot. B) Representative immunoblots (probed with total HTT antibody, MAB2166) of the cortex (CTX), striatum (STR), hippocampus (HIP) and cerebellum (CBL) from 3 month old BACHD animals following a unilateral intracerebroventricular (ICV) injection with either PBS or 150 μg HTT ASO. C) Relative quantification of mHTT levels in the brain following ICV injections with ascending doses of HTT ASO (15–150 μg) at 2 months shows dose-dependent lowering of mHTT in all brain regions 4 weeks post-treatment at 3 months of age (N=21 for PBS, N=17 for 15 μg HTT ASO, N=4 for 50 and 150 μg HTT ASO. Two-way ANOVA: treatment p<0.0001, brain region p<0.0001, interaction p=0.0110. Tukey’s multiple comparison test: HTT ASO ****p<0.0001 compared to PBS). D) Graphical experimental overview where 2 month old BACHD animals received bilateral IS injections of either PBS or ascending doses HTT ASO (0.15–15 μg). Four weeks after IS injections, treated animals were harvested and mHTT levels in different brain regions were quantified. E) Representative immunoblots of the CTX, STR, HIP and CBL following bilateral IS injections with either PBS or 0.5 μg HTT ASO. F) Relative quantification of mHTT in different brain regions following bilateral IS injections with ascending doses of a HTT ASO at 2 months of age shows potent lowering of mHTT that is restricted to the striatum with 0.5 μg HTT ASO at 4 weeks post-treatment (N=26 for PBS, N=8 for 0.15 μg HTT ASO, N=18 for 0.5 μg HTT ASO, N=21 for 5 μg HTT ASO, N=12 for 15 μg HTT ASO Two-way ANOVA: treatment p<0.0001, brain region p<0.0001, interaction p<0.0001. Tukey’s multiple comparison test: 0.5 μg HTT ASO CTX p=0.6082, STR ****p<0.0001, HIP p=0.9997, CBL p>0.9999 compared to PBS). G) Fluorescence in situ hybridization with a probe complementary to the HTT ASO sequence 72 hr after IS administration with 0.5 μg HTT ASO shows staining restricted to the STR. A dashed white line was added to highlight the boundaries of the striatum. IOD = integrated optical density.