In mice and humans the circadian rhythm of many biochemical reactions, physiology, and behavior is generated by a transcriptionaltranslation feedback loop (TTFL) made up of the so-called core clock genes/proteins. The circadian system interfaces with most signaling pathways including those involved in cell proliferation and inflammation. Cryptochrome (CRY) is a core clock protein that plays an essential role in the repressive arm of the TTFL. It was recently reported that mutation of CRY in p53-null mice delayed the onset of cancer. It was therefore suggested that CRY mutation may activate p53-independent apoptosis pathways, which eliminate premalignant and malignant cells and thus delay overt tumor formation. Here we show that CRY mutation sensitizes p53 mutant and oncogenically transformed cells to tumor necrosis factor α (TNFα)-initiated apoptosis by interfacing with the NF-κB signaling pathway through the GSK3β kinase and alleviating prosurvival NF-κB signaling. These findings provide a mechanistic foundation for the delayed onset of tumorigenesis in clock-disrupted p53 mutant mice and suggest unique therapeutic strategies for treating cancers associated with p53 mutation. hepatocellular carcinoma | inflammatory cytokine | extrinsic apoptotic pathway T he circadian rhythm in mammalian organisms is generated by a molecular clock comprising four gene/protein groups (1-5): The CLOCK (NPAS2) and BMAL1 transactivators and the cryptochrome 1 and 2 (CRY1/2) and the period 1 and 2 (PER1/ 2) repressors. CLOCK (or NPAS2) and BMAL1 make heterodimers, which act on the promoters of CRY1/2 and PER1/2 and activate their transcription. The CRY and PER proteins in turn, after a time delay of ill-defined mechanism, inhibit the CLOCK-BMAL1 transactivator and hence their own transcription to close the TTFL. This core TTFL directly or indirectly affects most cellular functions, and as a result, clock disruption by core clock gene mutation is expected to have serious repercussions at the cellular and organismal levels.We recently reported that mice of the p53 −/− Cry1 −/− Cry −/− (hereafter p53 KO Cry DKO ) genotype exhibited delayed onset of spontaneous cancer relative to p53 −/− (p53 KO ) mice (6). When analyzed for their response to genotoxic stress, it was found that p53 KO Cry DKO cells were more sensitive to UV-induced apoptosis than p53 KO cells even though they are identical in terms of DNA repair and DNA damage checkpoint functions (6). Therefore, we ascribed the reduced clonogenic survival of p53 KO Cry DKO cells upon UV irradiation, relative to p53 KO cells, to enhanced p53-independent apoptosis as a consequence of CRY mutation. We also suggested that the delayed onset of cancer and prolonged lifespan of mice of p53 KO Cry DKO genotype may have been caused by enhanced apoptosis of oncogenically transformed cells before giving rise to macroscopic tumors (6). In line with these findings we recently determined that p53 KO Cry DKO tumors are more responsive to oxaliplatin than p53 KO tumors (7). However, UV and oxaliplatin...