1. CAG repeat expansion: abnormal repetition of the CAG nucleotide sequence in the huntingtin gene. 2. Protein aggregation: accumulation of mutant huntingtin protein in neurons. 3. Basal ganglia degeneration: selective atrophy of brain structures involved in movement control. 4. Motor and cognitive impairments: jerky movements, cognitive decline, psychiatric symptoms. Here, HTT is linked to Cognitive impairment.