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The protein kinase Chk1 is required in the fission yeast Schizosaccharomyces pombe for delaying cell cycle progression in response to DNA damage. Chk1 becomes phosphorylated when DNA is damaged by a variety of agents, including the anti-tumor drug camptothecin. To further characterize the behavior of Chk1 in response to DNA damage, we used PCR-based mutagenesis of the chk1 gene coupled with in vivo gap repair to generate mutant alleles. Of 44 chk1 mutants recovered, six encode full-length proteins that confer a DNA damage-sensitive phenotype. All of the alleles render cells checkpoint-defective, but confer subtle differences in sensitivity to camptothecin or UV light. Mutant alleles were sequenced and served to identify regions of the protein that are critical for checkpoint function. [ABSTRACT FROM AUTHOR]