Schizosaccharomyces pombe hus1 promotes radioresistance and hydroxyurea resistance, as well as S and G2 phase checkpoint control. We isolated a human cDNA homologous to hus1, called HUS1. The major focus of this report is on a detailed analysis of the physical interactions of the HUS1-encoded protein and two other checkpoint control proteins, RAD1p and RAD9p, implicated in the cellular response to DNA damage. We found that HUS1p interacts with itself and the N-terminal region of RAD1p. In contrast, the C-terminal portion of the checkpoint protein RAD9p is essential for interacting with HUS1p and the C-terminal region of RAD1p. Since the N-terminal portion of RAD9p was previously demonstrated to participate in apoptosis, this protein likely has at least two functional domains, one that regulates programmed cell death and another that regulates cell cycle checkpoint control. Truncated versions of HUS1p are unable to bind RAD1p, RAD9p, or another HUS1p molecule. RAD1p-RAD1p and RAD9p-RAD9p interactions can be demonstrated by coimmunoprecipitation, but not by two-hybrid analysis, suggesting that the proteins associate as part of a complex but do not interact directly. Northern blot analysis indicates that HUS1 is expressed in different tissues, but the mRNA is most predominant in testis where high levels of RAD1 and RAD9 message have been detected. These studies suggest that HUS1p, RAD9p, and RAD1p form a complex in human cells and may function in a meiotic checkpoint in addition to the cell cycle delays induced by incomplete DNA replication or DNA damage.