At the transition from meiosis to cleavage mitoses, Drosophila requires the cell cycle regulators encoded by the genes, giant nuclei (gnu), plutonium (plu) and pan gu (png). Embryos lacking Gnu protein undergo DNA replication and centrosome proliferation without chromosome condensation or mitotic segregation. We have identified the gnu gene encoding a novel phosphoprotein dephosphorylated by Protein phosphatase 1 at egg activation. Gnu is normally expressed in the nurse cells and oocyte of the ovary and is degraded during the embryonic cleavage mitoses. Ovarian death and sterility result from gnu gain of function. gnu function requires the activity of pan gu and plu
