ATM and xeroderma pigmentosum: In an as yet experimentally untested mathematical model, the co-recessive inheritance theory by Lambert and Lambert predicted that diseases caused by mutations in DNA damage response genes may require more than one gene inactivation55, including ataxia telangiectasia (AT, associated with mutations in ATM), xeroderma pigmentosum and Cockayne syndrome (XP and CS, involving nucleotide excision repair genes), and cancer.