The 2.6 Angstrom crystal structure for human cyclin-dependent kinase 2(CDK2) in complex with CksHs1, a human homolog of essential yeast cell cycle-regulatory proteins suc1 and Cks1, reveals that CksHs1 binds via all four beta strands to the kinase C-terminal lobe. This interface is biologically critical, based upon mutational analysis, but far from the CDK2 N-terminal lobe, cyclin, and regulatory phosphorylation sites. CDK2 binds the Cks single domain conformation and interacts with conserved hydrophobic residues plus His-60 and Glu-63 in their closed beta-hinge motif conformation. The beta hinge opening to form the Cks beta-interchanged dimer sterically precludes CDK2 binding, providing a possible mechanism regulating CDK2-Cks interactions. One face of the complex exposes the sequence-conserved phosphate-binding region on Cks and the ATP-binding site on CDK2, suggesting that CKs may target CDK2 to other phosphoproteins during the cell cycle.