TOP1 and Spinocerebellar ataxia type 1 with axonal neuropathy: Spinocerebellar ataxia with axonal neuropathy type 1 (SCAN1), is caused by the homozygous H493R mutation in tyrosyl-DNA phosphodiesterase 1 (TDP1), an enzyme that initiates the repair of DNA topoisomerase 1 (TOP1)-induced SSBs by unlinking the TOP1 peptide from the break.