Expression of 2 key DNA damage sensors, ataxia telangiectasia mutated (ATM) (A), a serine-threonine kinase that recognizes DNA double strand breaks, and ataxia telangiectasia and Rad3 related (ATR) (B), a serine-threonine kinase that recognizes DNA replication stress, in TCMR and Normal. Another key sensor, PRKDC (DNA-PK), was also increased in TCMR (FC 1.22, P < 0.05). All 3 DNA damage checkpoint proteins activate p53 (C), a tumor suppressor protein that activates either the DNA repair pathway or apoptosis pathway. CDKN1A (P21) (D) is a cyclin-dependent kinase inhibitor that is activated by p53, thus linking DNA damage to cell cycle arrest.