The origin licensing repressor geminin is a unique bifunctional protein providing a molecular link between cellular proliferation, differentiation and genomic stability. Here we report the first molecular structure of human geminin, determined by EM and image processing at a resolution of 17.5 A. The geminin molecule is a tetramer formed by two dimers with monomers interacting via coiled-coil domains. The unusual structural organization of geminin provides molecular insight into its bifunctional nature.