Test Description

We use a PCR-based assay to detect trinucleotide repeat expansion (CAG) in the HD gene. The HD trinucleotide repeat number is variable and CAG expansions associated with clinical HD are defined as those of greater or equal to 38 repeats. 

(CAG)n expansion in the 5’ end of the Huntington disease gene has been associated with a risk of expressing/developing the clinical HD phenotype.  All sizes of < 26 CAG repeats have never been associated with an HD phenotype and sizes of 27-35 CAG repeats have not been associated convincingly with HD phenotype but have been demonstrated meiotic instability in sperm. Allele sizes of 36-39 have been associated with HD phenotype but with reduced penetrance. The absence of HD has not been documented in any individual with HD allele size of > 40 CAG repeats. 1,2,3,4

Sample Preparation: Sample Preparation (PDF)


Results of DNA analysis will be communicated in a written report to the referring professional.

ReasonDNA analysis is appropriate for the study of affected patients, for diagnostic confirmation in a presumed affected individual, and in pre-symptomatic adult at risk or prenatal testing situations.

Myers, RH (2004) Huntington’s disease genetics. NeuroRx 1:255-262.
2 The Huntington Disease Collaborative Research Group (1993) A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington’s disease chromosomes. Cell 72:971-983.
3 Andrew SE et al. (1994) A CCG repeat polymorphism adjacent to the CAG repeat in the Huntington disease gene: implications for diagnostic accuracy in predictive testing. Human Molec Genet 3:65-67.
4 ACMG/ASHG (1998) Statement: Laboratory Guidelines for Huntington Disease Genetic Testing. Am. J. Hum. Genet. 62:1243-1247.

updated 8/2/2012

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