Attempts to map the Restriction Point in the mammalian cell cycle typically involve stimulating quiescent cells with mitogens for increasing intervals, removing the stimulus and then determining the proportion of cells that reach S phase at some point later. This “fixed point” estimate assumes that further cell cycle commitment ceases as soon as the stimulus is removed. In fact, kinetic analysis shows that the probability of cell cycle commitment does not fall back to its initial low value, immediately after a pulse of mitogens, but may instead remain slightly elevated for some while afterwards, compared to the starting quiescent population. Thus, cells entering S phase after a brief exposure to mitogens are not those that pass the Restriction Point early. Rather, they represent cells that continue on to S phase as a result of this residual, low probability of cell cycle commitment. Instead, the mitogen-regulated process(es) affecting the probability of cell cycle commitment are much closer to the start of S phase itself. Since the acquisition of (apparent) mitogen independence is such a poor indicator of the timing of cell cycle commitment, it is argued that a better measure is the point of insensitivity to CDK4,6 inhibitors such as palbociclib, which indicates when hyperphosphorylation of the Retinoblastoma Protein, RB, ceases to be dependent on mitogen-signalling pathways regulating CDK4,6/cyclin D activity.

中文翻译：

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解说：定位限制点

绘制哺乳动物细胞周期中的限制点的尝试通常涉及用有丝分裂原刺激静止细胞以增加间隔，去除刺激，然后确定在稍后某个时间点达到 S 期的细胞比例。这个“固定点”估计假设一旦刺激被移除，进一步的细胞周期承诺就会停止。事实上，动力学分析表明，细胞周期定型的概率不会在有丝分裂原脉冲后立即回落到其初始低值，而是可能会在之后的一段时间内保持略微升高，与开始的静止群体相比。因此，在短暂接触有丝分裂剂后进入 S 期的细胞并不是那些提前通过限制点的细胞。相反，它们代表由于这种残留而继续进入 S 期的细胞，细胞周期承诺的可能性低。相反，影响细胞周期承诺概率的有丝分裂原调节过程更接近 S 期本身的开始。由于获得（明显的）有丝分裂原独立性是细胞周期承诺时间的一个糟糕指标，有人认为更好的衡量标准是对 CDK4,6 抑制剂（如 palbociclib）不敏感的点，这表明视网膜母细胞瘤何时过度磷酸化蛋白质 RB 不再依赖于调节 CDK4,6/细胞周期蛋白 D 活性的丝裂原信号通路。