Genetically, SCAs are grouped as repeat expansion SCAs, such as SCA3/Machado-Joseph disease (MJD), and rare SCAs that are caused by non-repeat mutations, such as SCA5.
Heterozygous mutations in the gene encoding β-III spectrin (SPTBN2) underlie SCA type-5 whereas homozygous mutations cause spectrin associated autosomal recessive ataxia type-1 (SPARCA1), an infantile form of ataxia with cognitive impairment.
While locus homogeneity with SCA5 is not formally excluded, we consider it rather unlikely on phenotypic grounds, and propose that this condition may represent an addition to the group of neurogenetic disorders subsumed under the rubric SCA.