SCA31 is caused by a 2.5- to 3.8-kb insertion of complex pentanucleotide repeats containing (TGGAA)n, which is located in overlapping introns of the BEAN1 and TK2 genes on chromosome 16q22.1 (Sato et al., 2009). The gene discussed is TK2; the disease is spinocerebellar ataxia type 31.