Cell cycle progression is dependent on the sequential activation of cyclindependent kinases (Cdks). For full activity, the cell cycle control protein Cdk2 requires phosphorylation of a conserved residue, Threonine 160, carried out by Cdk-activating kinase 1. The kinase associated phosphatase (KAP) is a human dual specificity protein phosphatase that dephosphorylates Cdk2 on Threonine 160 in a cyclin dependent manner. KAP binds to Cdk2 and dephosphorylates Threonine 160 when the associated cyclin subunit is degraded or dissociates. Fluorescence measurements show that Threonine 160 phosphorylation increases the affinity of Cdk2 for both histone substrates and ATP and decreases its affinity for ADP.