The activity of cyclin - Cdk complexes is affected by cyclin - dependent kinase inhibitors including P27kip1. P27kIp1 protein binds to cyclin D - Cdk4, cyclin E - Cdk2 and cyclinA - Cdk2 complex inhibiting their activities during the cell cycle.

• 目前发现的CDKI按结构和功能的不同可分成两大类：一类为Cip/Kip(CDK-interacting protein/kinase inhibition protein)，包括p21~(Cip1)、p27~(kip1)、p57~(kip2)等。