Gankyrin is an oncoprotein overexpressed in hepatocarcinoma cells that binds to the cell-cycle regulator CDK4 and the S6b ATPase subunit of the regulatory component of the proteasome. It belongs to the family of ankyrin-repeat proteins that appear to mediate protein-protein interactions in diverse biochemical processes. Gankyrin has been crystallized from polyethylene glycol solutions and diffraction data have been obtained from these crystals that extend to 2.1 A spacing.