other disorders

In other disorders (for example fragile X syndrome and
Friedreich ataxia) very large expansions occur, which prevent
transcription of the gene, and act recessively as loss of function
mutations. Other types of mutations occur occasionally in these
genes resulting in the same phenotype. In myotonic dystrophy the
pathological mechanism of the expanded repeat is not known. It
is likely that the expansion affects the transcriptional process of
several neighbouring genes. Juvenile myoclonus epilepsy is due to
the expansion of a longer repeat region (CCCCGCCCCGCG)
normally present in two to three copies in the gene promoter
region, expanding to 40 or more repeats in mutant alleles.
Trinucleotide repeat expansions have also been found in other
conditions (for example polysyndactyly from HOXD13 mutation),
where the pathological expansion shows no instability.