Byers et al. demonstrated that SCLC cells have an increased expression of the transcription factor E2F1 and its target genes, including EZH2, thymidylate synthase, mediators of apoptosis, and DNA repair proteins, such as poly [ADP-ribose] polymerase 1 (PARP1), checkpoint kinase 1 (CHK1), ataxia-telangiectasia mutated (ATM), and ataxia telangiectasia and Rad3-related protein (ATR) [35]. Here, CHEK1 is linked to small cell lung carcinoma.