HTT and Huntington disease: Huntington’s disease (HD, or Huntington’s chorea; named after George Huntington who was the first to describe it653) is caused by a single dominant allele of the huntingtin gene containing an expanded number of CAG repeats; the disease develops when the number of repeats exceeds 40.654 Of note, astrocyte-specific deletion of mutant huntingtin in mice globally expressing this gene eased disease symptoms and delayed its progression, indicating a role for astrocytes in the pathophysiology of the disease.655 Astrocytes in HD undergo morphological atrophy and loss of many homoeostatic functions.