To maintain genome stability in eukaryotic cells, DNA is licensed for replication only after the cell has completed mitosis, ensuring that DNA synthesis (S phase) occurs once every cell cycle(1). This licensing control is thought to require the protein Cdc6 (Cdc18 in fission yeast) as a mediator for association of minichromosome maintenance (MCM) proteins with chromatin(2-10). The control is overridden in fission yeast by overexpressing Cdc18 (ref. 11) which leads to continued DNA synthesis in the absence of mitosis(12). Other factors acting in this control have been postulated(13) and we have used a re-replication assay to identify Cdt1 (ref. 14) as one such factor. Cdt1 cooperates with Cdc18 to promote DNA replication, interacts with Cdc18, is located in the nucleus, and its concentration peaks as cells finish mitosis and proceed to S phase. Both Cdc18 and Cdt1 are required to load the MCM protein Cdc21 onto chromatin at the end of mitosis and this is necessary to initiate DNA replication. Genes related to Cdt1 have been found in Metazoa and plants (A. Whitaker, I. Roysman and T. Orr-Weaver, personal communication), suggesting that the cooperation of Cdc6/Cdc18 with Cdt1 to load MCM proteins onto chromatin may be a generally conserved feature of DNA licensing in eukaryotes.