The zebrafish p53M214K allele affects a protected amino acid residue within a region of the DNAbinding domain corresponding to a mutational hotspot in human cancer, creating a transactivation dead p53 plan. Immunoblotting was done using standard methods, and the antibodies used are defined in Supplemental Data. rget DDR kinases, none of these principles have been fastidiously examined in an animal product, and the underlying cell death process is uncertain. We produced p53 mutant zebrafish lines to be used in whole organism centered modifier genetic screens, to accelerate the development of physiologic ubiquitin conjugation independent DDRs. Zebrafish consistently recapitulate mammalian extrinsic and intrinsic apoptotic signaling. Homozygosity for p53e7 recapitulates critical characteristics connected with p53 deficiency in mammalian systems, including a strong tumefaction prone phenotype, insufficient G1 gate func-tion, and widespread cellular radioresistance. Here we identify chk1 like a gene whose loss maintains IR induced apoptosis in live p53 mutant zebrafish embryos, and then used in vivo epistasis explanations to dissect the underlying process. Unlike formerly identified p53 independent Endosymbiotic theory apoptotic pathways, which restore caspase 3 activation downstream of defective p53, Chk1 exhaustion invokes an ATM/ ATR caspase 2 axis that bypasses the mitochondrial and death receptor pathways. We show this Chk1 suppressed process might be induced in p53 deficient or BCL2overexpressing human cyst cells, giving a mechanistic explanation for the usage of Chk1 inhibitors in cancer treatment. A Morpholino Screen for Suppressors of p53 Radioresistance Identifies chk1 p53 mutant zebrafish embryos are refractory to DNA damageinduced cell death, as demonstrated by a very nearly complete absence of acridine orange labeling in the brain and spinal chord of live embryos analyzed 7. 5 hr after body IR delivered at 18 hr postfertilization. Morpholino antisense oligonucleotides were used by us to knock down two nonkinase checkpoint specialists and PF299804 1110813-31-4 G2 checkpoint kinases and eight zebrafish S in p53 mutant embryos. We considered the power of each knockdown to bring back cell death at 7. 5 hr post IR. Single knockdowns of genes examined, excluding plk2, plk3, and aurkb, radiosensitized p53 mutants with variable performance. Chk1 knockdown led to a staining pat-tern that closely resembled wild typ-e, although atm, atr, smg 1/atx, and chk2 deficiencies repaired only minor AO reactivity averaging 1848-52 of the p53 response. Enhanced IR induced cytotoxicity resulted particularly from chk1 knockdown since injections of a chk1 mismatch MO failed to radiosensitize p53 mutants, the chk1 MO resulted in a sturdy decline of the endogenous Chk1 protein pool, correlating with impaired Chk1 exercise, and a specific inhibitor of human Chk1, however not inhibitors of ATM or Chk2, phenocopied the effects of chk1 MO.