STRs database » HTT locus
Gene HTT
Location Coding in 4p16.3
Reference region chr4:3074876-3074933 (hg38)
chr4:3076603-3076660 (hg19)
Repeat unit CAG
Repeat type Standard
Distribution of repeats in a superpopulation
Overview of repeats

The normal length of CAG repeats in the HTT gene is below 27 repeats (Testa & Jankovic, 2019) while the low end has been shown to be 9 repeats (Read, 1993). Allele lengths 40 or more repeats causes Huntington’s disease (HD) and is fully penetrant, however, the disease may also occur in individuals with 36-39 repeats in which case they often encounter later age of disease onset and slower progression (Testa & Jankovic, 2019). The number of CAG repeats is negatively correlated with the age of motor onset (Gusella et al., 2014).
Intermediate alleles in the range of 27-35 have been associated with normal phenotype, but it has been shown that some of the transmission of these alleles were unstable and expanded to the pathogenic range (Semaka et al., 2010).

→ View STRipy's population-wide repeat data for HTT locus

Disease(s) and range of repeats
Disease Onset Inheritance Normal Intermediate Pathogenic
Huntington's disease (HD) Adult Autosomal dominant 9-26 27-35 ≥36