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Cytostatic factor denotes activity maintaining the meiotic arrest of an unfertilized vertebrate egg. Calcium-dependent release of arrest permits the regulated destruction of mitotic cyclins and completion of meiosis.

Frog oocytes and egg extracts turned cell-cycle regulation into a transferable biochemical activity that could be purified, removed, restored and followed through time. Xenopus laevis provides large oocytes and abundant eggs containing stockpiled proteins and maternal messenger RNA. Early embryonic cycles can alternate rapidly between DNA replication and mitosis with little or no intervening growth phases. This makes extracts powerful for reconstructing the core oscillator, while also limiting direct extrapolation to the checkpoints and growth control of a differentiated somatic cell.

An immature oocyte is arrested in meiotic prophase. Hormonal stimulation can induce germinal-vesicle breakdown and progression towards a mature egg. Cytoplasm from a mature donor can induce maturation after injection into an immature recipient, establishing that a cytoplasmic activity can drive nuclear events. The activity was termed maturation-promoting factor, later also called M-phase-promoting factor. The foundational transfer experiments were performed in another frog species, Rana pipiens, so they should not be misattributed specifically to Xenopus.^[ref] Xenopus subsequently made the activity accessible to extensive extract assays and biochemical purification. Serial transfer also suggested amplification, rather than transfer of a fixed stoichiometric amount of a structural nuclear component.

Crushing unfertilized Xenopus eggs yields cytoplasm in which added nuclei can undergo characteristic mitotic events. Assay-guided fractionation of this material purified an activity capable of promoting early mitotic changes, making a connection between a cytoplasmic functional assay and specific protein components.^[ref] MPF is the complex of CDK1, historically Cdc2, with a mitotic cyclin such as cyclin B. The cyclin is a regulatory partner; CDK1 is the catalytic kinase. Nuclear-envelope breakdown, chromosome condensation and histone H1 kinase assays give complementary outputs. Co-purification with kinase activity is stronger when supported by immunodepletion and restoration of the active complex, because crude H1 phosphorylation is not uniquely specific to CDK1.

Cyclin synthesis and destruction provide the changing input. Cycling extracts show cyclin accumulation followed by destruction around mitotic exit. Blocking protein synthesis prevents repeated cycles; supplying appropriate cyclin material can restore mitotic entry, while a nondegradable cyclin prevents normal exit. These interventions distinguish association from a causal requirement for periodic synthesis and destruction.^[ref] Cyclin destruction is mediated by the anaphase-promoting complex, an ubiquitin ligase, followed by proteasomal degradation. It is therefore regulated proteolysis, not dilution or loss of kinase protein, that helps reset the mitotic machinery. Destruction of additional substrates coordinates chromosome separation with exit.

The response is not determined by cyclin concentration alone. Inhibitory CDK1 phosphorylation by Wee1 opposes activation; Cdc25 removes inhibitory phosphates. Active CDK1 promotes Cdc25 and suppresses Wee1, creating positive feedback. Xenopus extract experiments approaching the same sustained cyclin dose from an initially low or high kinase state found different maintained outputs, providing evidence of cell-cycle hysteresis experiment.^[ref] Monitoring endogenous cyclin destruction and the persistence of the added nondegradable input controls the possibility that the two histories merely leave different amounts of cyclin. This explains how gradual accumulation can produce a decisive transition rather than requiring every molecule to respond linearly.

Unfertilized eggs also reveal regulated arrest. They remain in meiotic metaphase II through cytostatic factor activity, which restrains the machinery for cyclin destruction. A fertilization-associated calcium signal releases that arrest, permitting cyclin destruction and meiotic completion. Experimentally adding calcium to an appropriately arrested extract can mimic this switch; a sham addition and monitoring cyclin and nuclear responses separate a genuine cycle transition from nonspecific extract damage.

Egg extracts additionally allow replication licensing to be separated from subsequent initiation. Added sperm chromatin can assemble nuclei and replicate, while transfer between extracts and manipulation of kinase state test why DNA is copied once per cycle. Licensing is permitted in a low-CDK state, while initiation-associated high CDK activity helps prevent relicensing before mitotic exit. A completed round of DNA synthesis must be distinguished from repair synthesis or simple nucleotide incorporation by appropriate product and template controls. The central contribution of Xenopus is experimental causality: a conserved cyclin–kinase system, feedback-controlled transitions, regulated proteolysis and replication control can be dissected in a cell-free cytoplasm rather than inferred only from correlations in dividing cells.