However, cell cycle is negatively regulated by cyclin-dependent kinase inhibitor (CDKI) such as p27. p27 can bind to and inhibit the activity of cyclin-dependent kinases(CDK) thereby prevent cell cycle progression of late GI to S-phase transition.

• 已知细胞增殖周期受细胞周期素依赖性蛋白激酶抑制剂(cyclin-dependent kinase inhibitor，CDKI)如p27的负向调控。 p27结合并抑制细胞周期素依赖性蛋白激酶(cyclin-dependent kinase，CDK)活性，从而阻滞细胞增殖周期中DNA合成前期（G卜向DNA合成期旧）的转变。