Cell cycle regulator that is involved in modulating and adjusting cell proliferation according to the requirements of the developmental program (PubMed:10850494, PubMed:12919679, PubMed:17431409). Interacts with mitotic Cdk1-cyclin complexes to inhibit mitotic entry at the G2/M transition (PubMed:17431409). Likely to function by binding to the hydrophobic patch of cyclins to interfere with the interaction between the complex and certain Cdk1 substrates (PubMed:17431409). At the mid-blastula transition, involved in the cell cycle arrest in G2 of cycle 14 by delaying mitosis and thus reducing cell proliferation allowing cell fate specification and morphogenesis to take place (PubMed:12919679). Acts downstream or in parallel to the checkpoint regulator grp which is also required for the cell cycle pause at cycle 14 (PubMed:12919679). During gastrulation, delays mitosis in the ventral region of the embryonic mesoderm thus allowing invagination to be completed before cell division takes place (PubMed:10850494, PubMed:12919679, PubMed:17431409). Component of the Frs-CycA-Cdk1 complex composed of Z600, CycA and Cdk1. Interacts preferentially with CycA (via C-terminus) but is also able to interact (via C-terminus) with CycE (via C-terminus). Associates with chromatin in the syncytial blastoderm. Not detected until cycle 14 of embryogenesis when the cleavage cycles are completed (at protein level) (PubMed:12919679). Expression levels peak 15-30 min after cellularization and decrease during late cellularization and gastrulation (at protein level) (PubMed:12919679). Expressed until stage 9 in an area of dorsal cells that are probably the amnioserosa anlage (at protein level) (PubMed:12919679). At cycle 13, uniform expression throughout the cortex with expression levels increasing during stage 14 (PubMed:2478402). During cellularization expression localizes dorsally and posteriorly (PubMed:2478402). During gastrulation expression persists in dorsal regions of the embryo and by germ band extension expression is concentrated anterior to the amnioproctodeal invagination and posterior to the forming cephalic furrow (PubMed:2478402). Most embryos display premature mitosis resulting in defective invagination of the mesoderm during gastrulation. A small percentage of embryos also display an extra nuclear division prior to cellularization.