The autosomal dominant gene that is the cause of Huntington's disease has a mutation due to the expansion of a triplet sequence of (CAG) within the coding region of the gene. Since the sequence of the entire gene is now known, primers on either side of the tandem repeat sequence of (CAG)n can be used to amplify the region using the polymerase chain reaction and determine the size (number of times CAG is repeated).
The triplet repeat sequence can be classified as a STRPs, or "short tandem repeat polymorphisms" because it has the following properties:
- A short sequence (CAG) is repeated many times in tandem.
- The region is "polymorphic" (many forms), because there are many different alleles differing in the number of CAG repeats.
- Primers have been defined for amplification of the locus by the polymerase chain reaction.