MAPT and Huntington disease: • Cortex and striatum of HD patients: ↑ 4R/3R mRNA ratio (31, 32); ↑ 4R/3R tau protein (27, 31–33); ↓ 3R tau protein (27, 31). • Cortex and striatum of R6/1 and HD94 mice: ↑ 4R/3R mRNA ratio; ↑ 4R and ↓ 3R tau protein (31).