ATM and ataxia telangiectasia: PNKP has important roles in multiple pathways involved in DNA-damage repair, including single-strand breaks (SSBs) and double-strand breaks (DSB).14,21,22 The cellular response to these breaks involves a complex signaling network and the key activator of this network is the serine/threonine protein kinase ATM (AT mutated), which has been shown to target PNKP.23,24 Mutations in ATM are known to cause AT, another autosomal-recessive ataxia.