English: The structure of the Cdk2–cyclin A–p27 complex, as determined by X-ray crystallography, reveals that the inhibitor p27 (red) stretches across the top of the cyclin–Cdk complex. Only the amino-terminal region of p27 is shown in the structure. The amino- terminal end of this fragment contains an RXL motif that interacts with the hydrophobic patch of cyclin A. The carboxy-terminal end of the p27 fragment interacts extensively with the beta sheet of Cdk2, causing extensive disruptions to its structure; p27 also inserts into the ATP-binding site of Cdk2 and directly inhibits ATP binding. (PDB 1jsu) ^[1]