cDNA clones corresponding to the sequence for human early pregnancy factor were isolated from a human melanoma library and hybridized to DNA digested with four restriction enzymes obtained from twelve different subjects. Up to 20 cross hybridizing bands were observed. When hybridized to metaphase spreads from four different humans, significant signals were present in nine locations, on eight different chromosome arms. These results suggest that the early pregnancy factor gene is a member of a large gene family. The coding sequence for early pregnancy factor has a high degree of homology with the sequence for human chaperonin 10, and the gene family described here should contain the genes for both of these proteins.