Thus, rare neurodegenerative diseases such as ataxia with oculomotor apraxia 1 (AOA1) or spinocerebellar ataxia with axonal neuropathy 1 (SCAN1) are defective in genes (aprataxin (APTX) and tyrosyl-DNA phosphodiesterase 1 (TDP1), respectively) encoding enzymes involved in the repair of specific types of DNA single strand breaks. The gene discussed is APTX; the disease is cerebellar ataxia.