= Cell-cycle hysteresis experiment
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.<Building a cell cycle oscillator: hysteresis and bistability in the activation of Cdc2 by Joseph R. Pomerening and others>[pp. 3–4, Figure 3a–c: history protocols and H1 readout; p. 6, Methods: phosphorimaging]{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>.<Building a cell cycle oscillator: hysteresis and bistability in the activation of Cdc2 by Joseph R. Pomerening and others>[p. 3, control discussion; p. 4, Figure 3h; reported 60–90 minute approach to apparent steady state]{ref}
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