Gankyrin is a recently discovered oncoprotein that is a component of the 19S regulatory cap of the proteasome. Structurally, it contains a 33- amino acid ankyrin repeat that forms a series of alpha helices. It plays a key role in regulating the cell cycle via protein-protein interactions with the cyclin-dependent kinase CDK4. It also binds closely to the E3 ubiquitin ligase MDM2, which is a regulator of the degradation of p53 and retinoblastoma protein, both transcription factors involved in tumor suppression and found mutated in many cancers. Gankyrin also has an anti- apoptotic effect and is overexpressed in certain types of tumor cells such as hepatocellular carcinoma.