A three-dimensional view of CDK structure and activation. In monomeric Cdk2 (left; [PDB:1HCL]), the major C-helix (N-lobe) and the activation domain are close, ensuring that the catalytic pocket is inaccessible. Upon binding of cyclin A (right: [PDB: 1JST]), the C-helix and the activation domain are pulled apart - a configuration that is further fixed by phosphorylation of residue T160, making the catalytic pocket accessible for enzymatic activity. The position of the inhibitory Thr14 (T14) and Tyr15 (Y15) residues in the G-loop is also shown. Color code: CDK subunit, orange; cyclin subunit, green; purple indicates specific named protein domains. CDK, cyclin-dependent kinase.