Geminin, DNA replication inhibitor, also known as GMNN, is a protein in humans encoded by the GMNN gene. A nuclear protein present in most eukaryotes and highly conserved across species, numerous functions have been elucidated for geminin including roles in metazoan cell cycle, cellular proliferation, cell lineage commitment, and neural differentiation. One example of its function is the inhibition of Cdt1. Geminin is absent during G1 phase and accumulates through S, G2 phase and M phases of the cell cycle. Geminin levels drop at the metaphase-anaphase transition of mitosis when it is degraded by the anaphase-promoting complex.