PNKP and microcephaly: In addition, defects in the repair of ssDNA breaks also cause neurodegenerative diseases such as spino-cerebellar ataxia with axonal neuropathy 1, SCAN1 (OMIM #607251), caused by mutations in tyrosyl-DNA phosphodiesterase 1 (TDP1), and microcephaly, seizures and developmental delay, MCSZ syndrome (OMIM #613402), caused by mutations in polynucleotide kinase phosphatase (PNKP).