In Huntington chorea the causative mutation in the protein huntingtin is a

Expansion of trinucleotide repeats is an important genetic cause of human disease, particularly neurodegenerative disorders. There are three key mechanisms by which unstable repeats cause diseases: • Loss of function of the affected gene occurs in fragile X syndrome. In such cases the repeats are generally in noncoding part of the gene. • A toxic gain of function by alterations of protein structure as in Huntington disease and spinocerebellar ataxias. In such cases the expansions occur in the coding regions of the genes. • A toxic gain of function mediated by mRNA as is seen in fragile X tremor-ataxia syndrome. In this condition, the non coding parts of the gene are affected.