HTT and Huntington disease: The HD-causing mutation is a CAG repeat expansion located in exon 1 of the huntingtin (HTT) gene that is translated into an expanded polyglutamine stretch in the mutant-HTT isoform.1, 2 The HTT protein is involved in neurogenesis and the specification of neuronal and non-neuronal lineages,3, 4 vesicle transport,5, 6, 7 potassium-based cyclic AMP (cAMP) chemotaxis,8 and interactions with transcription factors.9