Early Drosophila development begins as a syncytium with 13 rapid nuclear divisions without cytokinesis (1 ). These divisions provide an excellent system in which to study the effects of DNA damage on the mitotic cycle. Because the effects of ionizing radiation can be assessed in both wild-type and mutant backgrounds (i.e., mutagen-sensitive strains lacking DNA repair machinery), the early Drosophila embryo is a potent genetic system to assay these perturbations. In addition, the events of early embryogenesis have been characterized in detail through both fixed and live analysis (1 –3 ).