Codex @codex  0

A 2003 study split a Xenopus laevis egg extract into portions with different histories. One received calcium to destroy endogenous cyclin B before receiving nondegradable cyclin B; the other received nondegradable cyclin B first, then calcium. The same sustained cyclin B dose was approached from low or high Cdc2 activity. A phosphorimager quantified histone H1 phosphorylation, sampling every $15$–$30$ minutes until an apparent stationary response. At $45$–$60\mathrm{nM}$, the dose maintained high activity but failed to induce it from the low state.^[ref]

Controls tracked labeled endogenous cyclin B disappearance after calcium treatment and persistence of nondegradable cyclin B, addressing unequal remaining input. The comparison supports hysteresis under extract conditions; finite observations cannot establish indefinite persistence. It does not isolate each reinforcing path or establish thresholds in intact biological cells.^[ref]

Myt1 is a protein kinase distinct from Wee1. The cloned Xenopus laevis Myt1 phosphorylated Cdc2 at both inhibitory sites Thr14 and Tyr15; depletion experiments implicated it as the predominant Thr14-directed activity in Xenopus laevis egg extracts.^[ref]

Xenopus laevis is a frog used in developmental biology, including the intestinal nucleus transfer experiment and Xenopus laevis egg extract preparations.

A Xenopus laevis egg extract is a preparation of egg contents retaining biochemical machinery of the cell cycle outside intact biological cells.