To identify new nonessential genes that affect genome integrity, we completed a screening for diploid mutant Saccharomyces cerevisiae strains that are sensitive to ionizing radiation (IR) and found 62 new genes that confer resistance. Along with those previously reported (Bennett et al., Nat. Genet. 29:426-434, 2001), these genes bring to 169 the total number of new IR resistance genes identified. Through the use of existing genetic and proteomic databases, many of these genes were found to interact in a damage response network with the transcription factor Ccr4, a core component of the CCR4-NOT and RNA polymerase-associated factor 1 (PAF1)-CDC73 transcription complexes. Deletions of individual members of these two complexes render cells sensitive to the lethal effects of IR as diploids, but not as haploids, indicating that the diploid G1 cell population is radiosensitive. Consistent with a role in G1, diploid ccr4Delta cells irradiated in G1 show enhanced lethality compared to cells exposed as a synchronous G2 population. In addition, a prolonged RAD9-dependent G1 arrest occurred following IR of ccr4Delta cells and CCR4 is a member of the RAD9 epistasis group, thus confirming a role for CCR4 in checkpoint control. Moreover, ccr4Delta cells that transit S phase in the presence of the replication inhibitor hydroxyurea (HU) undergo prolonged cell cycle arrest at G2 followed by cellular lysis. This S-phase replication defect is separate from that seen for rad52 mutants, since rad52Delta ccr4Delta cells show increased sensitivity to HU compared to rad52Delta or ccr4Delta mutants alone. These results indicate that cell cycle transition through G1 and S phases is CCR4 dependent following radiation or replication stress.