Romina Vuono and colleagues examined post-mortem brain samples from patients with HD and investigated the role of tau in the pathology of HD, and found that MAPT haplotypes affect the rate of cognitive decline in a large group of patients with HD, some mutant HTT aggregates co-localized with both the 3R- and 4R-tau deposits, and altered 4R:3R tau isoform ratio in Huntington’s disease brains [132]. This evidence concerns the gene HTT and Huntington disease.