“Huntington’s disease (HD) is a progressive brain disorder caused by a defective gene. This disease causes changes in the central area of the brain, which affect movement, mood and thinking skills. The cause of this disease is the defective gene identified in 1993 causes virtually all Huntington’s disease.

The Huntington gene defect involves extra repeats of one specific chemical code in one small section of chromosome 4. The normal Huntington gene includes 17 to 20 repetitions of this code among its total of more than 3,100 codes. The defect that causes Huntington’s disease includes 40 or more repeats. Genetic tests for Huntington’s disease measure the number of repeats present in an individual’s Huntington protein gene.”

Alzheimer’s Association (https://www.alz.org/alzheimers-dementia/what-is-dementia/types-of-dementia/huntington-s-disease)