In non-replicating cells, TOP1 cleavage complexes are primarily repaired by TDP1 (El-Khamisy et al., 2005), and TDP1 mutations cause the neurodegenerative disease spinocerebellar ataxia with axonal neuropathy (SCAN-1) (Takashima et al., 2002). The gene discussed is TOP1; the disease is spinocerebellar ataxia, autosomal recessive, with axonal neuropathy 1.