HTT and Huntington disease: Huntington's disease (HD) is an autosomal‐dominant, fatal, neurodegenerative condition which is caused by an abnormal CAG expansion located within exon 1 of the huntingtin gene.1 Because of its monogenic cause, HD gene carriers can be identified before the appearance of overt clinical signs, providing a privileged window through which to observe the preclinical pathogenic pathways in HD.