Huntington disease is an autosomal-dominant neurodegenerative disease of 
mid-life onset caused by expansion of a polymorphic trinucleotide (CAG) repeat. 
Variable penetrance for alleles carrying 36-39 repeats has been noted, but the 
disease appears fully penetrant when the repeat numbers are >40. An abnormal CAG 
repeat may expand, contract, or be stably transmitted when passed from parent to 
child. Assays used to diagnose Huntington disease must be optimized to ensure 
the accurate and unambiguous quantitation of CAG repeat length. This document 
provides an overview of Huntington disease and methodological considerations for 
Huntington disease testing. Examples of laboratory reports are also included.