Abstract: The present invention pertains to novel inhibitors of cyclin-dependent kinases (CDKs), particularly CDK/cyclin complexes, which inhibitors can be used to control proliferation and/or differentiation of cells in which the inhibitors are introduced. More specifically, the inhibitors of the invention are chimeric proteins which include CDK-binding motifs from two or more different proteins. For example, the subject chimeric proteins can be generated from the in-frame fusion of coding sequences from two different CDK inhibitor proteins, such as may be derived from fusion of coding sequences for an INK4 protein and coding sequences for a CIP protein. Chimeric proteins of the present invention have been observed to be more potent inhibitors of cyclin/CDK complexes than were either of the portions of the chimeric protein individually.