Geminin is a nuclear protein made up of about 200 amino acids, with a molecular weight of approximately 25 kDa. Geminin is a protein with a potential dual role during the metazoan cell cycle. Geminin was originally identified as an inhibitor of DNA replication and substrate of the anaphase promoting complex (APC). However, geminin also appears to play important roles in the progression of the cell cycle and cell proliferation, and is overexpressed in many cancerous tumors.

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 (APC).Recently, geminin has been found to be overexpressed in several malignancies and cancer cell lines.