Antibodies against the S. cerevisiae Ty3 endonuclease may be obtained by immunizing an animal, particularly a mammal such as a rabbit or mouse, with S. cerevisiae Ty3 endonuclease, harvesting antibody-producing cells from the immunized animal, fusing the cells with cultured immortalized cells to form hybrid cells, selecting hybrid cells which produce antibodies specific for S. cerevisiae Ty3 endon