The central role of cyclin-dependent kinases (CDKs) in cell cycle regulation makes them a promising target for studying inhibitory molecules that can modify the degree of cell proliferation. The discovery of specific inhibitors of CDKs such as polyhydroxylated flavones has opened the way to investigation and design of antimitotic compounds. A novel flavone, (-)-cis-5,7-dihydroxyphenyl-8-[4-(3-hydroxy-l-methyl)piperidinyl]-4H-l-benzopyran-4-one hydrochloride hemihydrate (L868276), is a potent inhibitor of CDKs. A chlorinated form, flavopiridol, is currently in phase I clinical trials as a drug against breast tumors. We determined the crystal structure of a complex between CDK2 and L868276 at 233-A resolution and refined to an Rfactor of 203%. The aromatic portion of the inhibitor binds to the adenine-binding pocket of CDK2, and the position of the phenyl group of the inhibitor enables the inhibitor to make contacts with the enzyme not observed in the ATP complex structure. The analysis of the position of this phenyl ring not only explains the great differences of kinase inhibition among the flavonoid inhibitors but also explains the specificity of L868276 to inhibit CDK2 and CDC2.Cell cycle progression is tightly controlled by the activity of cyclin-dependent kinases (CDKs) (1-3). CDKs are inactive as monomers, and activation requires binding to cyclins, a diverse family of proteins whose levels oscillate during the cell cycle, and phosphorylation by CDK-activating kinase (CAK) on a specific threonine residue (4, 5). In addition to the positive regulatory role of cyclins and CAK, many negative regulatory proteins (CDK inhibitors, CKIs) have been discovered (6), such as p16 (7-9), p21 (10-13), and p28 (14). Since