DNA damage activates cell cycle checkpoints in yeast and human cells. In the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe checkpoint-deficient mutants have been characterized, and the corresponding genes have been cloned. Searches for human homologs of S. pombe rad1, rad9, and hus1 genes identified the potential human homologs hRad1, hRad9, and hHus1; however, little is known about the roles of these proteins in human cells. The present studies demonstrate that hRad1 and hHus1 associate in a complex that interacts with a highly modified form of hRad9, but hHus1 and hRad1 do not associate with hRad17. In addition to being a key participant in complex formation, hRad9 is phosphorylated in response to DNA damage. Together, these results suggest that hRad9, hRad1, and hHus1 are central components of a DNA damage-responsive protein complex in human cells.