HTT and major depressive disorder: Huntington's disease (HD) is a neurodegenerative condition caused by a cytosine‐adenine‐guanine (CAG) repeat expansion in exon 1 of the huntingtin gene (HTT) (The Huntington's Disease Collaborative Research Group 1993) and is characterized by progressive motor, cognitive and psychiatric symptoms including involuntary movement disturbances, dementia and depression.