Huntington's disease is caused by a dominant allele of the gene that codes for the production of the huntingtin protein. This protein affects the development of many different tissues, including brain tissue. • The Huntington allele contains several repeats of the base sequence CAG, which codes for glutamine. • This results in a polyglutamine section in the synthesised protein. • A gene with more than 39 CAG repeats produces a protein that does not fold properly and does not function. • Symptoms of Huntington's disease usually first appear between the ages of 30 and 45 years. • There is no treatment for the disease, which is progressive and always fatal. • Some people with between 27 and 35 CAG repeats do not develop the disease, but may still pass on the Huntington allele to their children, who may develop the disease as the number of repeats tends to increase when gametes are produced.