Cutlet/CTF18 encodes an evolutionarily conserved protein that is crucial for germline development in Drosophila melanogaster. Loss of function of cutlet in the fly results in a sterile phenotype due to the failure of germline stem cells to proliferate. CTF18 was first identified in Saccharomyces cerevisiae as a sister chromatid cohesion factor that is essential for the faithful transmission of chromosomes during DNA replication. We have cloned and characterized the human and mouse CTF18 orthologs of the D. melanogaster gene, cutlet. We have demonstrated that CTF18 mRNA is expressed in human and mouse testis and ovary, and that CTF18 protein is expressed throughout the male and female germline of the mouse. We suggest a unique biological role for CTF18 in mammalian germ cell development based on its mammalian germline expression, high degree of evolutionary conservation, and role in DNA replication and chromosomal stability in yeast.