the p53 independent cell death causing DDR triggered by depletion can be a caspase3 independent apoptotic pathway. P53,chk1MO embryos lacked the IR induced G2/M gate, as would be expected from Chk1 loss. chk1 MO also fully radiosensitized p53e6 homozygotes and p53 morphants missing p53 protein, including in derivatives. Together, these results give in vivo evidence (-)-MK 801 that Chk1 destruction is sufficient to replace IR awareness to p53 mutant cells. Chk1 is essential for mouse and fly development, with homozygous null mutants succumbing to key cell cycle defects. We for that reason tested if the cytotoxicity of chk1 knock-down in zebrafish p53 mutants was firmly IR dependent. Indeed, chk1 depletion had no apparent impact on stability and normal zebrafish development, in both the p53 or p53 background. Western blots performed with an antizebrafish Chk1 antibody unmasked a knock-down of the protein. Yet chk1 morphants harbored extra levels of Chk1 activity, as shown by weak but persistent levels of phosphorylated Cdc2. These results show that transient exhaustion, rather than consistent complete loss, of Chk1 function, is tolerable by vertebrate cells in vivo and compatible with long haul organismal viability. Crucially, Plastid however, such temporary downregulation is enough to restore the IR induced cell death response in p53 mutants. Irradiated p53,chk1MO Embryos Undergo Caspase3 Independent Cell Autonomous Apoptosis Chk1 knockdown may restore awild kind reaction to IR or triggeradifferent cell death program in p53 mutants. Wefirst analyzedtwo hallmarks ofapoptosis: TUNELpositive DNA fragmentation and cleaved caspase 3 in embryos fixed at 7, to distinguish between these options. 5 hpIR. AO labeling of irradiated p53,chk1MO embryos correlated with high levels of pifithrin a TUNEL labeling throughout the CNS, similar to studies in irradiated p53 embryos. Multiple cells in-the CNS of p53 and Chk1 depleted p53 embryos also showed comparable ultrastructural manifestations of apoptosis. Surprisingly, but, while irradiated p53 embryos exhibited powerful immunostaining for active caspase 3, irradiated p53,chk1MO embryos did not and showed no increase in active caspase 3 levels compared to p53 simple mutants, that have been lacking both TUNEL and active caspase 3. To look for the mobile autonomy of the Chk1 antagonized route, we generated genetic chimeras. While p53,chk1MO cells grafted in-to p53 hosts often stained TUNEL positive after IR, nearby host cells didn’t. In the experiment, p53 cells transplanted in to p53,chk1MO hosts stayed TUNEL bad within an otherwise TUNEL positive environment.