The paper describes a novel protein, geminin, which is a 25 kDa protein that inhibits DNA replication and is degraded during the mitotic phase of the cell cycle. Geminin contains a destruction box sequence near its amino terminus, which is recognized by the anaphase-promoting complex (APC) and leads to its ubiquitination and degradation. In synchronized HeLa cells, geminin is absent during the G1 phase, accumulates during S, G2, and M phases, and is degraded at the metaphase-anaphase transition. Geminin inhibits DNA replication by preventing the incorporation of the MCM complex into the prereplication complex (pre-RC). The protein's inhibitory activity resides in an 80-amino acid region that includes a predicted coiled-coil structure. The biological function of geminin is to inhibit DNA replication at inappropriate times during the cell cycle, ensuring that replication occurs only once per cell cycle. The degradation of geminin at the metaphase-anaphase transition allows replication to proceed in the next cell cycle.The paper describes a novel protein, geminin, which is a 25 kDa protein that inhibits DNA replication and is degraded during the mitotic phase of the cell cycle. Geminin contains a destruction box sequence near its amino terminus, which is recognized by the anaphase-promoting complex (APC) and leads to its ubiquitination and degradation. In synchronized HeLa cells, geminin is absent during the G1 phase, accumulates during S, G2, and M phases, and is degraded at the metaphase-anaphase transition. Geminin inhibits DNA replication by preventing the incorporation of the MCM complex into the prereplication complex (pre-RC). The protein's inhibitory activity resides in an 80-amino acid region that includes a predicted coiled-coil structure. The biological function of geminin is to inhibit DNA replication at inappropriate times during the cell cycle, ensuring that replication occurs only once per cell cycle. The degradation of geminin at the metaphase-anaphase transition allows replication to proceed in the next cell cycle.