HTT and Huntington disease: The cause of HD has been shown to be a polymorphic CAG repeat expansion in the huntingtin (HTT) gene located on chromosome 4 that leads to abnormal degeneration of neurons within the striatum and cortex [123, 124] through several biological mechanisms including altered gene expression profile, disrupted mitochondrial and metabolic function, direct toxicity of the mutant protein, and aberrated ATP levels.