SCA31, one of the most common types of SCA in Japan, is caused by an insertion of complex pentanucleotide repeats containing (TGGAA)n in an intron belonging to brain expressed associated with NEDD4-1 (BEAN1) and Thymidine kinase 2 (TK2) genes, in chromosome 16q22.1 [34]. Here, TK2 is linked to autosomal dominant cerebellar ataxia.