The protein products of several rad checkpoint genes of Schizosaccharomyces pombe (rad1+, rad3+, rad9+, rad17+, rad26+, and hus1+) play crucial roles in sensing changes in DNA structure, and several function in the maintenance of telomeres. When the mammalian homologue of S. pombe Rad9 was inactivated, increases in chromosome end-to-end associations and frequency of telomere loss were observed. This telomere instability correlated with enhanced S- and G2-phase- specific cell killing, delayed kinetics of γ-H2AX focus appearance and disappearance, and reduced chromosomal repair after ionizing radiation (IR) exposure, suggesting that Rad9 plays a role in cell cycle phase-specific DNA damage repair. Furthermore, mammalian Rad9 interacted with Rad51, and inactivation of mammalian Rad9 also resulted in decreased homologous recombinational (HR) repair, which occurs predominantly in the S and G 2 phases of the cell cycle. Together, these findings provide evidence of roles for mammalian Rad9 in telomere stability and HR repair as a mechanism for promoting cell survival after IR exposure.