Overview of repeats
The normal range of repeats in ATXN2 gene has been found to be as less as 13 repeats and reaching up to 31 repeats (Pérez et al., 2009). In the vast majority of cases the healthy allele is 22, followed by less frequent 23 (Sanpei et al., 1996). Expansions at least 35 repeats are known to cause spinocerebellar ataxia type 2 (SCA2) disease (Sanpei et al., 1996) and alleles from 32 to 34 are considered as intermediate ones (Riess et al., 1997. However, there is a report of an individual with 33 CAG repeats who developed SCA2 at the age of 86, indicating that 33 repeats can be pathogenic as well and cause late onset of the disease (Fernandez et al., 2000).
In addition, Elden and colleagues have reported about intermediate alleles in the length of 27-33 which were associated with increased risk for ALS (Elden et al., 2010).