p21 effectively inhibits Cdk2, Cdk3, Cdk4, and Cdk6 kinases (Ki 0.5-15 nM) but is much less effective toward Cdc2/cyclin B (Ki approximately 400 nM) and Cdk5/p35 (Ki > 2 microM), and does not associate with Cdk7/cyclin H

• p21 --inhibit--> Cdk2
• p21 --inhibit--> Cdk3
• p21 --inhibit--> Cdk4
• p21 --inhibit--> Cdk6