ATR and small cell lung carcinoma: Association of TREX1 methylation with SCLC response to the ATR inhibitor Cpd45 may be consistent with the previously reported TREX1 upregulation in cancer cells in response to DNA damaging agents and to UV light, and roles in sensitivity and response to DNA-damaging agents, DNA repair, and/or DNA degradation after drug treatment [50–52], including degradation of ssDNA fragments by TREX1 and TREX1 interaction with PARP1 (poly(ADP-ribose) polymerase-1) in response to DNA damage [77, 78].