onset neurodegenerative disorders

A number of late onset neurodegenerative disorders (for
example Huntington disease and spinocerebellar ataxias) are
associated with expansions of a CAG repeat sequence in the
coding region of the relevant gene, that is translated into
polyglutamine tracts in the protein product. These mutations
confer a specific gain of function and cause the protein to form
intranuclear aggregates that result in cell death. There is
usually a clear distinction between normal- and disease-causing
alleles in the size of their respective number of repeats and no
other types of mutation are found to cause these disorders.