The rad21 gene of Schizosaccharomyces pombe is involved in the repair of ionizing radiation-induced DNA double-strand breaks. The isolation of mouse and human putative homologs of rad21 is reported here. Alignment of the predicted amino acid sequence of Rad21 with the mammalian proteins showed that the similarity was distributed across the length of the proteins, with more highly conserved regions at both termini. The mHR21(sp) (mouse homolog of Rad21, S. pombe) and hHR21(sp) (human homolog of Rad21, S. pombe) predicted proteins were 96% identical, whereas the human and S. pombe proteins were 25% identical and 47% similar. RNA blot analysis showed that mHR21sp mRNA was abundant in all adult mouse tissues examined, with highest expression in testis and thymus. In addition to a 3.1-kb constitutive mRNA transcript, a 2.2-kb transcript was present at a high level in postmeiotic spermatids, while expression of the 3.1-kb mRNA in testis was confined to the meiotic compartment. hHR21sp mRNA was cell cycle regulated in human cells, increasing in late S phase to a peak in G2 phase. The level of hHR21sp transcripts was not altered by exposure of normal diploid fibroblasts to 10 Gy ionizing radiation. In situ hybridization showed that mHR21sp resided on chromosome 15D3, whereas hHR21sp localized to the syntenic 8q24 region. Elevated expression of mHR21sp in testis and thymus supports a possible role for the rad21 mammalian homologs in V(D)J and meiotic recombination, respectively. Cell cycle regulation of rad21, retained from S. pombe to human, is consistent with a conservation of function between S. pombe and human rad21 genes.