ABSTRACT
Trinucleotide repeat diseases can be broadly grouped into two classes, depending on the location and degree of expansion of the unstable repeats which cause them. Diagnostic testing for trinucleotide repeat diseases is carried out by determining the number of repeats carried by a potentially disease causing gene. However, as the difference of a single trinucleotide repeat in the measured size of an allele can have enormous diagnostic/prognostic implications, diagnostic testing for trinucleotide repeat diseases is critically dependent on ability to very accurately determine the size of DNA fragments. Different approaches have been used by different laboratories for the diagnosis of trinucleotide repeat diseases, such as fragile X syndrome and myotonic dystrophy, where disease allele may contain thousands of repeats. However, as with other trinucleotide repeat diseases, due to highly variable phenotype and age-of-onset associated with any particular CAG repeat number, size of the CAG repeat cannot be used to predict either the severity or the age-of-onset of the disease.
