The product of the Schizosaccharomyces pombe rad9+ gene is required for cell cycle arrest at the G2 checkpoints in response to incompletely replicated or damaged DNA. We have identified a human cDNA from an infant brain library that is a structural homolog of S. pombe rad9+, by searching the dBest data base for sequences similar to the fission yeast gene. The human gene encodes a 391-amino acid long, 42,520-Da protein that is approximately 25% identical and 52% similar to the yeast protein. The human and yeast gene products demonstrate partial conservation of function, as the human cDNA can rescue to different degrees the sensitivity of S. pombe rad9::ura4+ cells to the DNA synthesis inhibitor hydroxyurea and gamma rays, as well as the associated checkpoint controls. These results suggest an underlying conservation of the molecular mechanisms of S and G2 checkpoint control pathways in most if not all eukaryotes. Fluorescence in situ hybridization using a fragment of the corresponding human genome as a probe, in conjunction with PCR reactions employing DNA from human X rodent somatic cell hybrids, has localized the gene to human chromosome 11q13.1-13.2. This region contains a number of tumor suppressor loci, and based on the biology of checkpoint control genes, HRAD9 should be considered a strong candidate for one of them.