p21 is a cyclin-dependent kinase (Cdk) inhibitor that is activated by p53 in response to DNA damage. It selectively inhibits Cdks involved in the G1 and S phases of the cell cycle, such as Cdk2, Cdk3, Cdk4, and Cdk6, but is less effective against Cdc2/cyclin B and Cdk5/p35. p21 does not associate with Cdk7/cyclin H. Overexpression of p21 arrests cells in G1, indicating its role in cell cycle regulation. p21's inhibitory activity is enhanced by cyclin binding, a property shared with p27, another Cdk inhibitor. p21 and p27 both inhibit Cdk2 and Cdk4, but p27 has slightly different kinase specificities. In normal diploid fibroblasts, most active Cdk2 is associated with p21, and exogenous p21 can fully inhibit it. These findings suggest that p21 functions as an inhibitory buffer, with its levels determining the threshold kinase activity required for cell cycle progression. p21 is associated with the majority of active Cdk2 in normal fibroblasts, and its levels can set the threshold for Cdk activity in vitro. p21's inhibitory activity is enhanced by cyclin binding, and it shows selectivity for kinases involved in G1 decisions. p21 overexpression arrests cells in G1, consistent with its role in mediating the p53 checkpoint function. p21 does not associate with Cdk7/cyclin H and shows no inhibitory activity toward Cdk5/p35 complexes. These findings indicate that p21 is not a universal inhibitor of Cdks but shows selectivity for kinases that regulate G1 decisions.p21 is a cyclin-dependent kinase (Cdk) inhibitor that is activated by p53 in response to DNA damage. It selectively inhibits Cdks involved in the G1 and S phases of the cell cycle, such as Cdk2, Cdk3, Cdk4, and Cdk6, but is less effective against Cdc2/cyclin B and Cdk5/p35. p21 does not associate with Cdk7/cyclin H. Overexpression of p21 arrests cells in G1, indicating its role in cell cycle regulation. p21's inhibitory activity is enhanced by cyclin binding, a property shared with p27, another Cdk inhibitor. p21 and p27 both inhibit Cdk2 and Cdk4, but p27 has slightly different kinase specificities. In normal diploid fibroblasts, most active Cdk2 is associated with p21, and exogenous p21 can fully inhibit it. These findings suggest that p21 functions as an inhibitory buffer, with its levels determining the threshold kinase activity required for cell cycle progression. p21 is associated with the majority of active Cdk2 in normal fibroblasts, and its levels can set the threshold for Cdk activity in vitro. p21's inhibitory activity is enhanced by cyclin binding, and it shows selectivity for kinases involved in G1 decisions. p21 overexpression arrests cells in G1, consistent with its role in mediating the p53 checkpoint function. p21 does not associate with Cdk7/cyclin H and shows no inhibitory activity toward Cdk5/p35 complexes. These findings indicate that p21 is not a universal inhibitor of Cdks but shows selectivity for kinases that regulate G1 decisions.