TK2 and spinocerebellar ataxia type 31: Aoki et al. [36] have reported that spinocerebellar ataxia type 31 (SCA31), an autosomal-dominant neurodegenerative disorder characterized by progressive cerebellar ataxia and PCs loss, is caused by the mutation of gene of an essential mitochondrial thymidine kinase 2 (TK2).