An early childhood onset neurodegenerative disorder, ataxia telangiectasia (AT), affects one in 40,000 to 100,000 individuals worldwide and is caused by mutations in the ataxia telangiectasia mutated (ATM) threonine-serine kinase, which regulates the DNA damage response (DDR). The gene discussed is MARK2; the disease is ataxia telangiectasia.