Abstract : There are few reports describing the role of p53-dependent gene repression in apoptotic cell death. To identify such apoptosis-associated p53 target genes, we used the pro-oxidant plant-derived drug thymoquinone and compared p53+/+ and p53-/- colon cancer cells HCT116. The p53 wild-type (wt) status correlated with more pronounced DNA damage and higher apoptosis after thymoquinone treatment. A significant up-regulation of the survival gene CHEK1 was observed in p53-/- cells in response to thymoquinone due to the lack of transcriptional repression of p53. In p53-/- cells, transfection with p53-wt vector and CHEK1 small interfering RNA treatment decreased CHEK1 mRNA and protein levels and restored apoptosis to the levels of the p53+/+ cells. p53-/- cells transplanted to nude mice treated with thymoquinone up-regulated CHEK1 expression and did not undergo apoptosis unlike p53+/+ cells. Immunofluorescence analysis revealed that the apoptosis resistance in p53-/- cells after thymoquinone treatment might be conveyed by shuttling of CHEK1 into the nucleus. We confirmed the in vivo existence of this CHEK1/p53 link in human colorectal cancer, showing that tumors lacking p53 had higher levels of CHEK1, which was accompanied by poorer apoptosis. CHEK1 overexpression was correlated with advanced tumor stages (P = 0.03), proximal tumor localization (P = 0.02), and worse prognosis (1.9-fold risk, univariate Cox regression; Kaplan-Meier, P = 0.04). We suggest that the inhibition of the stress response sensor CHEK1 might contribute to the antineoplastic activity of specific DNA-damaging drugs.