Lethal phase is during embryogenesis/larval stages. 40% of geminin^k03202b/Df(2R)ST1 animals die before hatching and the rest die during larval development (third instar larvae are not seen). The overall S phase patterns of geminin^k03202b/Df(2R)ST1 animals appear normal during early embryogenesis and cells exit from S phase at the appropriate developmental time. Stage 11 embryos undergoing cycle 16 show an increased number of mitotic cells. Approximately 30% of these cells show anaphase defects, including anaphase chromosome bridges and chromosomes apparently cut by the cytokinetic contractile apparatus. The mitotic spindle appears normal in cells with anaphase defects. In stage 16 geminin^k03202b/Df(2R)ST1 embryos, endoreplication occurs in more cells in the midgut and dorsal cell domains than in wild type and continues in the hindgut and Malpighian tubules, where it has ceased by this stage in wild-type embryos. More cells than normal in the central nervous system are in S phase.