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.