Understanding molecular genetical background of hereditary chorea has recently been progressed so far. Triplet repeat expansion diseases including, Huntington disease, in which CAG expansion has been identified in the IT-15 or Huntingtin gene, and Huntington disease like-2, in which CTG expansion in junctophilin-3 (JPH3) gene occurs, causes selective degeneration of striatum in the brain. Octapeptide repeat expansion in the prion gene in Huntington disease like-1 has been also identified. Neuroacanthocytosis syndromes including McLeod syndrome and chorea-acanthocytosis cause acanthocytosis in the red blood cells and chorea due to the degeneration of caudate nucleus in the brain. The XK gene on the X chromosome is mutated to lose its function in McLeod syndrome. CHAC gene coding chorein is mutated to lead loss of function in chorea-acanthocytosis. Selective degeneration in the striatum, especially in the caudate nucleus might be associated with the molecular cascade of expanded polyglutamine or polyleucine or octapeptide and the loss of function of the XK protein and of chorein protein.